GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK, NORWAY

BY

DOR O T HY WYCKO F F

WITH 5 MAPS AND 19 PHOTS. I N THE TEXT

Introduction.

Location and Geography of Telemark.

Telemark Fylke 1 is the section of south-central Norway which

includes the drainage basins emptying into the Skiensfjord2, as well as the land around the large !akes N isservatn 3 and Fyres vatn. These !akes dra in in to the Nid elv 4, a river flowing to the sea through Aust-Agder, the next fylke to the southeast. Telemark is irregular in shape, but the greater part of it Iies between 59° and 60o north latitude, and 71/zo and 91/zo longitude east of Greenwich.

The narrow coastal strip of Telemark includes several towns -Skien, Porsgrund, Kragerø and Brevik. In the northern part, how­ever, the only towns of considerable size are Notodden, on the Heddalsvatn and Rjukan, in the Vestfjorddal5 west of Tinnsjø6.

Telemark is not crossed by any great continuous valleys, like those which form the main thoroughfares of many other sections of Norway. The drainage system is complex, with many large and small !akes, bogs and streams, cutting the country into a confused network of valleys. The farms are most! y grouped in grender 7, and communication with grender in nearby valleys, and with the outside world, is in many places roundabout and difficult. The popu­lation of the moors and uplands is sparse. There are a few little

l fylke -- a political division of the country, corresponding roughly to state.

2 -fjord- as used in Norway means any enclosed arm of the sea - a bay or

inlet; it is also used for large !akes.

3 -vatn - water, lake.

4 -elv - river.

� -dal- valley.

G -sjø -- sea, (large) lake.

grend (pl. grender)- a gro up of farms, neighborhood; less compact !han a vill age.

Norsk geo\. tidsskr. XIII. l

2 DOROTHY WYCKOFF

farms in some of the upper valleys; but over wide tracts of the mountains there are no permanent settlements at all. For a few weeks in midsummer, peasants live in the ru de seters 1, pasturing their cattle on the high levels; in September they return to their homes in the valleys.

The mountains of southern Telemark are relatively low and, where they do not rise above the tree-line (900-1000 meters above sea leve!), are for the most part covered with forests of spruce, pine and birch. Farther north, the mountains show steep sides and rounded tops of bare rock; but some of the higher ones- e. g. Gaustafjell2 ( 1883 meters3) and Vindeggen ( 1492 meters 4)- are topped by steep narrow ridges, where snow Iies through most of the year.

Early Descriptions.

Telemark is a part of Norway in which systematic mmmg was very early begun, the copper and silver mine at "Sundzberg" -probably Sundsbarm in Seljord - having been chartered in 1524.

A few geologica1 items are included in LUND's "Description of Upper Telemark in Norway"5. WILLE'S "Description of the Parish of Seljord"6 (which was at the time considered a model of local de­scription, and is still a valuable source book for those interested

in the antiquities, customs, and superstitions of Telemark) also contains chapters on natura! history, in which the author gives his observations on rocks, soils, mines, caves, etc.

The first geologist to visit the regions was the German, NAUMANN, who travelled and studied in Scandinavia in 1821-22; but since he went through Telemark in May, when there was still much snow, his observations are too incomplete to be of great value. His book was published in 1824 7.

I seter - a summer dairy-farm, like the chalets of Switzerland. 2 �fjell - mountain. 3 New measurement (1923-24) by Den Geografiske Opmåling, Oslo. Sheet 35. 3,9. 4 HELLAND, AMUND , Norges Land og Folk, Bratsberg Amt, p. 27, Kristiania, 1900. 5 LuND, jo HAN MICHAEL, Forsøg til Beskrivelse over Øvre Tele marken i Norge,

Kjøbenhavn, 1785. o WILLE, HANS jACOB, Beskrivelse over Sillejords Præstegield, Kjøbenhavn,

1786. 7 NAUMANN, C. F., Beitrage zur Kentniss Norwegens, Leipzig, 1824.

•

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 3

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4 DOROTHY WYCKOFF

Meanwhile the Norwegian geologist KEILHAU had also travelled through Telemark, and published a pa per 1 dealing particularly with the rocks which were later to be ca lied the "Telemark Forma ti on". As WERENSKIOLD remarks2, "The fact that KEILHAU so early began to study the geology of Telemark certainly had a great influence on all his peculiar conceptions of the mountainbuilding of Scandinavia -in few places does one find such thorough and intense metamorphism, at !east in southern Norway. All kinds of rocks seem to grade into each other; this supported KEILHAU's 'Transmutation doctrine'3".

Since KEILHAU's time numerous geologists have visited the region. The more important papers which endeaver to unravel the question of the origin and history of the "Telemark formation" may be divided in to two groups:

l. Those dealing with the relation of the whole "Telemark formation" to the pre-Cambrian formations in other parts of Scandi­navia or Fennoscandia.

2. Those concerned with the separation of different members within the "Telemark formation", and the determination of their relations to each other.

The Scandinavian pre-Cambrian.

In Finland and in southern Sweden pre-Cambrian rocks are continuously exposed over great areas. The earliest Swedish geo­logists distinguished two types: to the west, the "iron gneiss'', a strongly compressed metamorphic red granite containing small grains of magnetite (whence its name); and farther east, a series of less altered granites, porphyries and tuffs -- the "granite-leptites". In the zone of intense metamorphism between the two, and in the "iron gneiss'' near this zone, occur intrusive masses of hyperites -- elon­gated patches which are parallel to the general strike of the gneiss. For some time it was believed that the "sharp" contact between these

t KEILHAU , B. M., Om de Skandinaviske Formationers Anden Suite: Mag.

f. Naturv. bd. l, 1823.

2 WERENSKIOLD, WERNER, Om Øst-Telemarken: N. G. U. nr. 53, Å rbok for ]909,

p. I l , 1910.

3 KEILHAU 'S theories are full y expressed in his book: Gaea Norwegica, Kristi­

ania, 1 838, !850.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 5

two areas must be marked by a fault line 1 : but with more detailed mapping the contact was found to be not "sharp" but transitional.

DE GEER2 then proposed the explanation that the highly meta­morphosed border zone is the result of an ancient period of mountain­building: the "iron gneiss" and the "granite-leptite" series were originally rocks of the same type, but the former has been subjected to more intense metamorphism than the latter. The hyperites were forced up in to fissures form ed during the mountain-building; the fact that they now appear less metamorphosed than the surrounding rocks is due to their great thickness which made them resistant to crushing, except peripherally. This hypothesis was combatted by T6RNEBOHM 3, who argued that the greater metamorphism of the "iron gneiss" in­dicated that it must be the older formation, probably representing the "basement complex" upon which the "granite-leptites" had been laid down. DE GEER's explanation, however, won a general acceptance among Swedish geologists, though the age of the mountain chain (which DE GEER ascribed to the Algonkian) is still a matter of dispute4. In any case, it is possible that such a mountain chain had some relation to the continental landmass which is supposed to have existed to the south of Scandinavia in earl y Cambriam or previous time 5.

BuGGE 6 has attempted to carry over this system of correlation in to the Norwegian pre-Cambrian. In Norway the study of the most ancient rocks is more difficult than in Sweden. The two areas where pre-Cambrian rocks are exposed in southern Norway are separated from each other by the "sparagmite formation'' (the earliest Paleozoic sediments), and by the region of the Oslofjord, with its

I NATHORST, A. G., Ett forsok att fOrklara orsaken til! den skarpa gransen mellan

sodra Sveriges vestra och ostra urterritorium: G. F. F., bd. 8, p. 95, 1886.

BACKSTROM, H., Vestanåfaltet, en pet rografisk studie: K. V. A ., hand!.,

bd. 29, nr. 4, p. 104, 1897.

2 DE GEER, G., Om A lgonkisk Bergveckning inom Fenn oskandias g ransområden:

G. F. F., bd. 21. nr. 198, 1899.

3 ToRNEBOHM, A. E., Discussion , Jan. 4, 1900, of Prof. DE GEER's lecture of

De c. 7: G. F. F., bd. 22, p. 116, i 800.

4 TORNEBOHM, loe. cit.

HoGBOHM, A. G., Pre-Cambrian Geology of Sweden: Bull. Geo!. Inst., Upsala,

bd. 10, p. 59, 1910.

s BRøGGER, W. C., Norges Geologi: Norge 1814-1914, p. 199,200.

6 BuGGE, ARNE, Et Forsøk på lnddeling av det Syd-no rske Grundfjeld: N. G. U.,

n r. 95, 1 92'2.

6 DOROTHY WYCKOFF

faults and post-Silurian igneous rocks; and they are likewise se­parated from the pre-Cambrian rocks in western and northern Norway by the central mountain chain, of Caledonian age. It has therefore become the custom to speak of a num ber of "formations" : the " Bamble formation", the "Telemark formation", the "Modum forma­tion" and the gneisses of Smålenene.

The Smålenene gneisses, southeast and east of Oslo, are con­tinuous with the gneisses of southwest Sweden, and have always be en considered to belong wi th them 1•

West of the Oslo region, strongly metamorphic gneisses are found in two areas: o ne at Kongsberg, extending northeastward to Gran and Lake Mjøsen; the other in a stri p along the south coast, from Bamle to Kristiansand S. To the west and north of these gneisses Iies the "Telemark formation" with its less strongly metamorphosed quartzites, leptites, and granites; and to the southwest the granites of southern Norway. Both these gneisses and the gneisses of Små­lenene are accompanied by the characteristic hyperites which are found in the "iron gneiss'' in Sweden, and the strikes may be inter­preted in agreement with the old mountain chain postulated by De Geer. The distribution of these formations is shown on Map l (based on maps given by BuGGE).

More detailed mapping, however, in the region of Kongsberg

and Bamle, revealed the fact that the contact between the gneisses and the "Telemark formation" northwest of them is marked by a fault line with a conspicuous friction breccia 2. This fact, and the character of the rocks themselves, led BuGGE to the conclusion that the gneisses of Bamle and Kongsberg are really older than the rocks of inner Telemark. His interpretation of the history of this region may be briefly summarized as follows:

The oldest gneiss is the "Kongsberg formation", of undetermined origin; the next, the "Bamle formation", which contains schists and quartzites, some of them at !east of sedimentary origin. These two formations were subjected to a thorough metamorphism, at the close of which fissures were formed and filled by a series of gabbro rocks and a later series of granites.

I BRøGGER, W. C., op. cit., p. 109.

2 BuGGE, ARNE, En Forkastning

nr. 130, 1928.

det Syd-norske Grunnfjell: N. G. U.,

Scb.le

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 7

l::·:.·:·.iPo�tCa.mb,.ib.rl 1·''·-··�J G,.anites Is 1 1 1 1 1\ Gneiss . · · · · J J orm a. tlons . � \ ' , ,j . _

1 !Spa.ra._s�mite 1·:. · :l·;;�r;t�i!l� jl':o;? /l Hype,.ites

�1111111111111 ;��:t�ne leptites��?�p�e;;oids � h������;, et a.nØ quc)l'txl tes. Telem�,.K ?IKe Ma.p 1: Geolo3i c Ma.p o! 5ouThern N orwa.y a.nd Sweden.

o�sed on rnaps by APne Ou file . 19Z�. l9lB.

Map. l .

8 DOROTHY WYCKOFF

Upon these Archean formations were laid down the supracrustal rocks of the "Telemark formation", in to which the "Telemark granite'', with its accompanying and following dykes of pegmatites and basic rocks, was later intruded. This period of igneous activity was also one of stress, of fissure-formation, and finally of faulting: the land on the east was elevated so that the oldest gneisses stood on a leve! with, or were pushed over, the "Telemark formation''. Along the southern edge the overthrust was less marked, and it dies out to the west.

The highland formed by this uplift was worn down by prolonged erosion, and the materials removed from it were deposited as the series of sediments - now arkose, sandstone and slate - which the Norwegian geologists have named the "Sparagmite formation", at the very base of the Paleozoic 1 •

Peneplanation was practically complete by the beginning of Paleozoic time, and with the encroachment of the sea, the later Cambrian and Silurian formations were laid down upon the old land­surface: they are found res ting upon the Archean gneiss2 at Gran and in Bamle, and upon the so-called "post-Telemark gran i te" (the last intrusive in the "Telemark formation") upon the Hardangervidda3.

The "Telemark formation".

DAHLL 4 was the first to recognize that there are in Telemark rocks of two different kinds: l) stratified rocks, the quartzites and schists (among which the hornblende schists were included) and 2) igneous rocks, the granite mass in lower Telemark, which he considered to be of later date.

t GoLDSCHMIDT, V. M., Geologisch-petrographische Studien im Hochgebirge des Siidlichen Norwegens: Vid. Selsk. Skr. I, nr. 19 p. 7, 1 9 1 2.

VoGT, TH ., Forholdet mellem Sparagmitsystemet og det Marine Underkambrium ved Mjøsen: Norsk Geol. Tids. bd. 7, 1923.

2 HoLTEDAHL, OLAF, and ScHETELIG, jAKOB, Kartbladet Gran: N. G. U. nr. 97, 1 923.

BRØGGER, W. C., and ScHETELIG, jAKOB, Geologisk Kart over Kristiania­feltet: N. G. U., 1 923.

3 GOLDSCHMIDT, V. M., op. cit. 4 DAHLL, TELLE F , Om Telemarkens Geologi, Nyt Mag. f. Naturv. bd. Il, 1860.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 9

This is the interpretation generally accepted today, but it has not been undisputed. The Swedish geologist TORNEBOHM 1 designated the granite as "granite gneiss'' and believed it to be of sedimentary origin: he evident! y considered it in connection with the gneisses of Sørland, and this interpretation was the more tempting, inasmuch as it fitted into the scheme he had already set up for the gneisses of Vermland in Sweden. The Norwegian geologists, however, received the proposal without enthusiasm, and continued to regard the "granite gneiss" as an igneous rock. REUSCH considered the granite of un­certain age2, but possibly older3 than the sediments. But other investigators have found very convincing proofs that it is younger, probably an intrusive laccolith: The gran i te grows finer grained and shows differentiation along the border4• 5; in places dykes of it in trude the country rock 5; and "contact o res", most ly of co p per, occur along the contact 6.

The succession of the other members of the "Telemark forma­tion'' has not yet been completely worked out. WERENSKIOLD 7 has reviewed previous investigations, and given a generalized map (see Map 2) showing the distribution of various rock types. His tentative scheme for the stratigraphy of the region is as follows:

l T6RNEBOHM, A. E., Några notiser från en geologisk resa i Telemarken: G. F. F. bd. Il, p. 46, 1889.

2 REU SCH, HANS, Geologiske Iagttagelser fra Telemarken, Indre Hardanger, Numedal og Hallingdal: Kr. Vid. Selsk. Forh., nr. 2, 1896.

3 IDEM, Om Telemarkens Fjeldbygning: Naturen, bd. 27, p. l, 1903. 4 WERENSKIOLD, WERNER, Om Øst-Telemarken: N. G. U., nr. 53, Årbok for

1909, pp. 16-21, 1910. � BRøGGER, W. C., Die Mineralien der Siidnorwegischen Granitpegmatitgange:

Vid. Selsk. Skr. I nr. 6, pp. 1 1-- 12, 1906. li VoGT, J. H.L., Norske Ertsforkomster: Archiv, f. Mat. og Naturv., bd. 9, 12.

WERENSKIOLD, WERNER., Om Øst-Teiemarken: N. G. U., nr. 53, Årbok for 1909, pp. 48, 49, 1910.

IDEM, Tekst til Geologisk Kart over Strøkene mellem Sætersdalen og Ringe­rike: N. G. U., nr. 66, 1912.

lO DOROTHY WYCKOFF

Rock type

7. Granites (youngest)

Found in

Tinn, Atrå and Venås­fjell

6. Basic instru- All parts of Telemark sives (of the same age, or slightly younger than 5)

5. Telemark Granite

4. Quartzite

3. Grarzulite

2. Quartzite

l . Porphyroids (oldest)

Southern Telemark as far north as Kviteseid; also the northwestern part toward the Hard­angervidda

Lifjell and Blefjell

Heddal and Hovin

Svartdal, Brattefjell, Vindeggen and Gausta

Tuddal and Tinn

Described as

A light granite containing predominant microcline, with orthoclase and quartz; acces­sory biotite.

Gabbros, "greenstones", horn­blende schists.

A strongly metamorphic red­dish granite, containing pre­dominant albite, with ortho­clase, microcline and quartz.

White and grey quartzites, schists and conglomerates. Thickness in Blefjell ca. 1500 m. in Lifjell possibly greater.

Strongly compressed schists, porphyritic or with quartz "eyes". In places the y show an apparently bedded struc­ture; they may be tuffs. At Notodden occur a few thin beds of marble.

Red and white quartzites, with arkose, conglomerate and schists. Thickness 2000-3000 m.

Porphyries, tuffs and volcanic agglomerates, in places schistose.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK I l

CJI] Gra,ei,CI�>y. L:J Telemb.rK glacra.l !Ill S''"'nite '

� Sje�itic rock) � Qvo.r-tLife oJ ' o TRe Q::.to Olefjell on .

r J•O') Lijjetl

� Gra.nite � Grd.tlvlites . ( linr5 indica!e etrrKe)

��:<:rf B.s.sit: r-oc.Ks � s�artdal o a,uartl-1te

� Porttroids (lin � rndiCd.te � tdKe)

� Gnei��� Ko�sbe g

;l). 5trd(e and drp O-\ �uart�rte�

'3ca.le:

,-. -. -. -.

l l i AREA l l OF l l ;Mo.p.3.;

10 l'S kn"t

Mo..p 2: Geolor,;ic Ma.p oJ E.a.st Telem�rK, Norwa.y. f3�sed on descriP'trons �nd ma.ps of W.WerensK1old, 1909,1912.

Map. 2.

12 DOROTHY WYCKOFF

The Mt. Gausta Region. Physiography.

The area under discussion in this paper comprises about sixty square kilometers, lying between Vestfjorddal and Tuddal. The central part of the area is occupied by Mt. Gausta. The deep Vestfjorddal is cut into the mountain plateau just north of Mt. Gausta, and a more irregular country of hills and valleys slopes away to the south. The topography is in late youth or sub-maturity, with many youthful features superimposed by glaciation.

The Vestfjorddal, running in general from west to east, is a glacially deepened valley; its floor at the city of Rjukan is only 300 meters above sea leve!, and 700-800 meters below the leve! of the mountain plateau to the north. Vestfjorddal is the !argest of four tributary valleys which converge to the head of Tinnsjø, a large lake filling a trough valley similar in form to the fjords of western Norway. This must have been a great glacial confluent basin. The sides of Vestfjorddal are extremely steep (Photograph 1 ) , the bottom flat and covered in the lower part by glacial gravels and sand; its mouth, submerged beneath the waters of Tinnsjø, forms the Vestfjord - the bay on the west side of Tinnsjø - from which the valley takes its name. Farther up, where the cover of loose material is thinner, the U-shaped valley shows a distinct gorge incised in its bottom (Photograph 2). AHLM.ANN 1 points out that "the sides of the gorge are smoothed and striated by ice, which shows that, for the most part, it must have formed before the last glaciation; its con­siderable size points to the same fact''. The main valle y was eroded during the first great glacial epoch, after which occured an interglacial period of such length that the river had time to form the gorge. During the last glacial epoch, the ice action was less severe: it fol­lowed, but did not destroy, interglacial stream valleys. At the head of the gorge is a steep cliff over which the river rushes with an uninterrupted dro p of 106 meters. This is the famous Rjukan foss 2.

t AHLMANN, H. W., Geomorphological studies in Norway: Geografiska Annaler,

bd. l, pp. 27-31, 1919. 2 -foss = waterfall. Rjukan means "smoking", a name given because of the

clouds of spray. The fall is six kilometers above the city of Rjukan, which

has grown up with the development of the water-power in this valley. The

Norsk Hydro's series of power stations together make up one of the !argest

electric power plants in the world.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 13

Directly south of the city of Rjukan, Mt. Gausta rises abruptly

from the mountain plateau; the peak is 1883 meters a bo ve sea leve!,

700-800 meters above the plateau, and almost 1600 meters above the valley bottom. Mt. Gausta is the highest mountain in southern Norway: it is visible from jotunheim, 200 kilometers away to the north, and it is said also to be visible, in clear weather, from the sea, just outside Kragerø. The peak is a sharp, narrow ridge, and AHLMANN 1 says, "it is strikingly distinguished from its surroundings.

Phot. 2. Vestfjorddal, looking east toward the city of Rjukan, showing the gorge

within the U-shaped valley, and the sharp peak of Mt. Gausta on the south, rising above the general leve! of the plateau.

In contrast to the even, glacially eroded side of Vestfjorddal, and to the rolling mountain-expanse around, also distinctly affected by ice, Gausta shows no trace of glacial erosion. Its sides are deeply fur­rowed by such small ra vi nes 2 as are found in regions that have long lain exposed to subaerial denudation, and scarcely anywhere does any solid rock come to the surface. The rock has been dissolved3

1 Op. cit., pp. 31-32.

2 These ravines hold the snow nearly all summer, giving the peak a peculiar

striped appearance, entirely different from !hat of any other mountain in the

vicinity (see Photograph 1).

3 "disintegrated" would perhaps be a more accurate word: the rock is a hard

quartzite, and mechanical weathering is certainly meant.

14 DOROTHY WYCKOFF

by atmospheric weathering, and the whole mountain is a ruin. These features indicate that Gausta lay exposed above the inland ice con­siderably longer than its surroundings: possibly during the whole last glacial epoch it was a nunatakk 1."

Southeast of the peak, Mt. Gausta stretches out in a lower, flatter ridge, some l ,5 km. broad, rising slightly again to the lesser summits ( 144 7 and 1529 meters high) Gaustaknærne -· "the kne es of Gausta".

On the southwest side the mountain is bounded by Gausdal, a rather narrow, glacially scoured valley occupied by a string of small !akes. This valley slopes gently southeast, from 929 meters at the highest lake to 888 meters at the lowest. Southwest of Gausdal rises another ridge, running southeast, parallel to Mt. Gausta, reaching its highest elevation ( 129 3 meters) in Bonsås; and south west of this again Iies another valley occupied by !akes- Heddevatn and Bonsvatn.

On the northeast side of Mt. Gausta a broader, shallower valley, most of which is filled by the lake Heddersvatn (at an altitude of 1 1 36 meters), merges eastward into a rugged plateau with many small domed summits. Heddersfjell, just northeast of Heddersvatn, is one of the more conspicuous ( 146 1 meters in height).

Heddersfjell, Mt. Gausta, and the ridges southwest of Gausdal are all made up of quartzite; the form of the peaks and ridges is somewhat modified by the dip of the strata.

South of Gaustaknærne and Bonsås the topography becomes much more irregular, dropping away to the south in a series of lower hills and ridges. The cause of this irregular topography seems to be, in part, the differing resistance to erosion of the different rocks here exposed, but no doubt the distribution of glacial till has also been important. A conspicuous glacial feature is Råen, a great morainal ridge of boulders running southeast from Mt. Gausta. It appears as a distinct line for the first four or five kilometers, though the whole region is thickly covered with til!. It seems to have formed as a sort of media! moraine where the ice converged again after being divided by the nunatakk of Mt. Gausta.

The river from Gausdal pl unges through li short gorge in to Reisjåvatn (at an altitude of662 meters). This lake occupies a valley ha ving the appe­arance of a small cirque. It is of interest to note that this gorge, like the one studied by AHLMANN in Vestfjorddal, shows glacial striæ on its sides.

l See also REUSCH, HANS, Geologiske lagttagelser i Telemarken o. s. v., Kr. Vid.

Selsk. Forh. nr. 2, 1896.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 15

The !akes west and southwest of Bonsås also empty into Reisjå­vatn through a small gorge and a series of falls. The general direction of drainage in this region in southeast, though there are many bogs and tarns interlinked in a complicated pattern. The outlets of Kovstul­vatn and of Heddersvatn, as well as that of Reisjåvatn, all follow this general southeasterly direction, and all enter Bjårvatn at Tuddal. Bjårvatn drains southeast, and its waters eventually reach Heddals­vatn just above Notodden.

These small valleys and basins, like the deep Vestfjorddal, are excavated in the rocks which underlie the quartzite - a series of altered volcanic Javas and tuffs. These rocks are of varying resistance: Reisjåfjell and Buarås are made up of a rather massive quartz por­phyry, which seems to be more resistant than the thin-bedded tuffs which Iie to the southeast. Volcanic porphyries appear again in Åråhøvda. There are also resistant masses of intrusive rocks, such as the gabbro which forms Hesjåbutinden. Other instances of structural control of the topography will be noted in later sections.

Geology.

Distribution and Classification of Rock Types.

Within this area are exposed rocks of three main types, all of pre-Cam brian age:

l. A lower series of schistose or massive metamorphic rocks, called by WERENSKIOLD 1 "porphyroids" and thought to be the oldest part of the "Telemark formation". These are of volcanic origin and include both tuffs and Javas.

2. An upper series of sedimentary rocks- arkose, quartzite and slate - overlying the volcanics with a large angular unconformity. These rocks are part of the "Svartdal quartzite" of WERENSKIOLD and earlier writers, and are probably the oldest of the Telemark quartzites.

3. Igneous rocks- granite and gabbro with accompanying "green­stone" sheets and dykes - intruding both the volcanics and the sediments. The granite and the gabbro are approximately contem­poraneous, and are similar to WERENSKIOLDS "Telemark granite" (found in south and west Telemark) and the basic rocks associated with it. The distribution of these types is shown on Map 3.

l WERENSKIOLD, WERNER, Om Øst-Telemarken : N. G. U., nr. 53, Årbok for

1909, p. 48, 1910.

16

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M~p3

DOROTHY WYCKOFF

LEGE ND

t Km. Topography from Norwe,9iAn Su•vey m"ps. Geo lo gy by llWyc.l<ojf 1:12.9,19 30.

Map. 3.

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The V olcanics.

The "porphyroids" are exposed to a depth of at !east 1000 meters in Vestfjorddal, and cover large areas on the mountain plateau to the north and east, and in the more deeply dissected country to the south of Mt. Gausta.

In Vestfjorddal, at the town of Rjukan, the porphyroids appear to be volcanic Javas. They are for the most part porphyritic, with small shining phenocrysts of feldspar and (more rarely) quartz, in a dense, dark grey groundmass, which in places shows flow-structures. They are massive to schistose in character. The dip of the schistosity is steep toward the south or southeast, but it is impossible to say whether this is the actual dip of a series of flows or sheets. In the lower part of the valley (5-6 km. beyond the northeast boundary of Map 3) this structure seems rather to be induced by pressure from the intrusive mass of gran i te in Atrå: the schistosity grows more and more pronounced toward the contact zone.

To the west (4 -6 km. beyond the boundary of Map 3) the Javas disappear from the valley, giving place to a series of bedded tuffs and volcanic agglomerates. These dip in general rather steeply south­east, but the direction and steepness vary from place to place. At the Rjukanfoss (Photograph 3) the layers are very fine and thin, striking N 60-70" E, dipping 70-90" southeast. On Maristi (the new road) above the fall, bedding is not apparent, but the fragmenta! character is striking, some of the fragments being 70-80 cm. or even more, in length.

South of Mt. Gausta the porphyroids are again exposed beneath the sediments. Here a more thorough study of their relations was attempted, and a smaller area mapped in detail (Map 4 ). Besides thin-bedded tuffs very similar to those found at the Rjukanfoss there are Javas of several different types.

T h e T u f f s .

Description: In the Reid the tuffs appear as evenly banded rocks having a strike in a general northeasterly direction and a steep dip (Photograph 4). In some layers the bands are minutely fine- 15 -20 to the cm.; in other places single apparent! y homogeneous bands may be 10-20 or rarely up to 30 cm. in width. The thinner bands are darker in co l or --- gra y or black; the thicker o nes a dull yellow-

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 19

ish or greenish grey; when much weathered, both are paler in co! or. There is a slaty cleavage, most pronounced where the layers are thinnest; in the thick layers the fracture is rough to conchoidal. The texture is so fine that little can be made out with the naked eye (or a hand lens) save very small flakes of biotite, which are more

abundant in the darker layers. Under the microscope, a

thin section from one of the thicker layers shows unmistake­ably the characteristics of a tuff, though a tuff that has undergone extensive alteration. In ordi­nary light, the rock appears to be made up of innumerable narrow, angular or curved particles of a clear, colorless material, embedded in a very fine-grained groundmass. The appearance is similar to that of some modern tuffs, made up principaily of threads and shards of glass, with a few broken crystals ( Photograph 5).

Between crossed nicols, this appearance is completely lost: a few of the fragments are seen to be feldspars, but most of the smaller bits almost merge in to the groundmass; the whole rock is composed of an aggregate of small an-

Phot. 3. Thin-bedded tuffs at Rjukanfoss.

hedra! crystals ( Photograph 6). Not a trace of the original glass remains: the shards are entirely made up of secondary al bite. This mineral has a peculiar tex ture; ill-defined laths Iie in superimposed fan-like groups, so that the extinction of the aggregate as a whole is wavy, patchy, or incomplete. With high magnification, thin albite twin­ning lamellae can be made out in places, but much of the material is untwinned; however, the index of refraction (n < balsam) and the positive character of the biaxial interference figure indicate albite.

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Besides these fragments of altered glass, there are broken crystals of microcline. These are irregular grains without recognizable crystal outlines. Microcline twinning is commonly present, Carlsbad twinning rarely. Some of these crystals show partial replacement by albite. In such cases the albite extends from the edges inward, but without definite crystal boundaries; it is in all respects similar to the albite described in the preceding paragraph. In ordinary light, it is readily distinguishable from the microcline by its higher index of refraction.

Phot. 4. Exposure of bedded tulfs. (Length of hammer 65 cm.).

There are a few other fragments made up of a dense felt of minute laths of a colorless mineral having the index of albite, to­gether with rounded grains of a colorless mineral with a much higher index and very low birefringence. The grains are so extremely small that it is impossible to get complete optical data for these two mine­rals, but such aggregates are undoubtedly composed of albite and zoisite, derived from a calcic or intermediate plagioclase feldspar by "saussuritization ".

Fragments of quartz are small and few, high! y irregular in form; some contain almost submicroscopic round inclusions.

There are also a few grains that may be regarded as much altered mafic minerals. These are composed of zoisite, with black iron oxide (altered in part to limonite and leucoxene); al bite, sericite

22 DOROTHY WYCKOFF

Phot. 5. Thin section of tuff, showing shards of (altered) glass.

(Plane polarized light).

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 23

Phot. 6. Same field of view as Phot. 5.

(Crossed nicols).

24 DOROTHY WYCKOFF

and quartz may also be present. It is idle to speculate what the original mineral might have been, for no traces of its form or cleavage remain.

The dense groundmass in which these fragments Iie is composed of fine anhedral grains of sericite, quartz and feldspar. Twinning in the feldspar is rarely to be observed, but the indices of refraction, especially where the grains Iie in contact with balsam at the edges of the section, indicate that both albite and microcline are present, albite predominating. Quartz is still more abundant than feldspar, and sericite the most abundant of all.

Scattered through the whole thin section are large flakes of biotite. This is strongly pleochroic: X pale straw yellow, Y, Z dark olive to reddish brown; X Y = Z. The figure in convergent light appears uniaxial and is negative. A striking characteristic of this biotite is the rather large dark pleochroic haloes. The mineral at the center of the hal o is too small to be identified with certainty; its high index of refraction and birefringence indicate zircon. The ragged form of the biotite, including patches of the groundmass material, points to a later introduction by hydrothermal solutions. The titaniferous magnetite or ilmenite which is closely associated with the biotite has pro ba bly the same origin; it is in subhedral grains and to a great extent altered to leucoxene. The small euhedral zircons may have been introduced at the same time: it is noticeable that some of them cut right across fragments of albitized glass.

In a few places occur rounded areas of granular quartz; these are ill-defined, merging imperceptibly into the fine texture of the groundmass at the edges. The coarser mosaic of quartz grains at the center contains also biotite and magnetite grains. Such minerals might have been deposited as a filling in some small hollow in the original porous ash, or they may be later replacements of other mine­rals. Actual veins of granular quartz also occur in cracks.

The original (probably vitric) tuff has evidently undergone rather intense metasomatic alteration, in which the following sequence can be traced:

I. Devitrification of the glassy fragments and/or replacement by albite; at the same time, replacement of part of the potash feldspar by albite. Formation of much sericite in the groundmass - the potash no doubt derived chiefly from the potash feldspar.

2. Introduction of the iron oxides, biotite and quartz by a later phase of hydrothermal activity.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 25

Phot. 7. Thin section of Tuff, showing ''varve"-like bands. (Plane polarized light).

This sequence is one that is to be observed, either in part or in a still more extended form, in all the volcanic rocks of this area. There will later be occasion to discuss it in more detail.

A thin section of one of the more finely banded layers shows a quite different appearance. At first glance, in ordinary light, the

most striking feature is the peculiar appearance of the fine bands

(Photograph 7): not on! y do coarser and finer bands alternate, but within a single band there is a gradual transition from coarser to

finer particles, with an abrupt change to coarser again at the bottom of the next band. The whole is similar to the "varve" structure of certain fine sediments.

26 DOROTHY WYCKOFF

The most important constituents are quartz and sericite; feldspar is alm ost or entirely lacking. In the coarser bands, the re is a gre at amount of biotite (similar to that described above), which shows in places an incipient alteration to green chlorite. The finer bands have almost no biotite, but a much greater amount of sericite, and this sericite shows a sub-parallel arrangement in the direction of the bands. Rarely a few small angular fragments of quartz occur, but in general the interlocking texture of these minerals suggests a rather extensive recrystallization. The parallelism of the sericite indicates that this recrystallisation took place, in part at !east, under pressure. Later cross-cutting cracks contain vein-quartz and limonite.

The intimate field relations between rocks of this and of the preceding type make it seem almost certain that this material also is a volcanic tuff, sorted into thin beds by some depositional agent - wind or water - and then subjected to further alteration. The sequence suggested for the preceding rock must here be a little extended: l) H ydrothermal altera ti on of the original particles of ash, with formation of sericite at the expense of the potash feldspar. 2) Dynamic metamorphism of the rock, with partial recrystallisation of the constituents, and paraBel orientation of the sericite. 3) Renewed hydrothermal activity, with introduction of biotite: the biotite is more abundant in the coarser layers, doubtless because those layers, being more porous, offered greater opportunity for the circulation of hydrothermal solutions. The Jack of any orientation in the biotite flakes indicates that this period followed, rather than preceded, the period of dynamic metamorphism. 4) Another period of slighter dynamic movement, with formation of new cracks and joint planes. 5) Still another period of hydrothermal activity, with introduction of chlorite (partially replacing the biotite), and filling of cracks with iron oxides and quartz.

The dynamic metamorphism of these rocks is, as matter of fact, more thorough than the preceding descriptions would lead one to believe. In a series of thin sections, hardly one can be found which shows no effects of pressure, and in many the original textures are so completely obliterated that it would be impossible to make out the history of the rock except by comparison with other specimens showing progressive metamorphism.

Those showing the recognizable textures of tuffs grade, even within a single thin section, into fine-grained micaceous rocks. Che-

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 27

mical and mechanical changes seem to advance simultaneously: first the flattening, then the obliteration, of the curved and angular frag­ments; at the same time, a decrease in the amount of feldspar and an increase in the amount of sericite; as the sericite increases in amount, its tendency to parallel orientation becomes more apparent; and the few remaining quartz fragments, having escaped chemical alteration, now show undulatory extinction, indicative of the pressure they have undergone.

A study of a similar series of the finely banded rocks shows much the same sequence of events. Any attempt to explain the banding as a result of pressure alone must be given up in the case of specimens showing fine, evenly graded hands, with abundant seri­cite developed and oriented at an angle of 30-40c from the bedding plane. In a few cases the bands themselves have been crumpled on a fine scale by tangential pressure, but the distinct identity of each band, coarser or finer in texture, is preserved.

From this same series of thin sections the sequence of the hydrothermal changes may be somewhat more exactly determined. In two or three cases the biotite as well as the sericite shows a tendency to parallelism. While this may be a purely local effect, due for example to pressure from intrusive mass of igneous rock, it suggests the possibility that the period of hydrothermal activity which introduced the biotite may have followed closely upon, and indeed been in part contemporaneous with, the period of dynamic metamorphism which orientated the sericite.

Many thin sections show biotite more or less altered to chlorite. but a few show biotite which is little or not at all altered, with larger flakes of chlorite also present. This chlorite is aphrosiderite 1:

it is apparently uniaxial, negative (rarely positive). The negative form has pleochroism: X colorless to pale yellow, Y, Z blue green, X < Y = Z; the positive form has X, Y blue green, Z pale yellow to colorless, X =Y> Z. The index of refraction, determined by the immersion method is � = 1.630; x and 1 could not be directly deter­mined, but the birefringence is very low, strong dispersion giving anomalous purple-blue colors. In one or two cases this chlorite is intimately associated with ve in quartz filling cracks in the rock; and

J Determined from diagram given by A. N. WINCHELL: Elements of Optical

Mineralogy, v. 2, p. 376, 1927.

28 DOROTHY WYCKOFF

in the rare cases where the biotite shows a para!lel orientation, the chlorite (in larger flakes) is quite without any such arrangement. It is thus evident that the chlorite was introduced later than the biotite, during a subsequent period of hydrothermal activity.

Granular quartz, either as veins or as fillings of rounded or oval ca vi ties, is present in all the thin sections; ca leite may be asso­ciated with it, filling the very center of the vein or cavity, indicating a final stage of carbonate solutions.

Interpretation and history: The fragmenta!, banded or bedded character of these rocks, both in the field and in the thin section, distinctly indicates a sedimentary origin. It is tempting to the imagination to think that we may have here some of the very earliest sedimentary rocks, deposited when the first rains fell upon the earth's surface. HADDING 1 has discussed the significance of the pre-Cambrian sediments, and unhesitatingly assigns some of the Swedish leptites to this earliest series of sediments, pointing out that the first sediments would be formed directly from the volcanic ash which must have accumulated in great quantities on the waterless surface of the earth. They would have a fragmenta! character, but would differ little in chemical composition from igneous rocks, and would respond like them to various later metasomatic changes.

The delicate "varve" -like bands observed in parts of this forma­tien would seem to indicate rapidly alternating conditions of deposition. SEDERHOLM2 is of the opinion that apparently similar "varved" sediments found in the Bothnian (Archean) formation in Finland represent seasonal or annua! variations in rainfall or run-off which may have been established even at that early time. KtNDLE3 however has demonstrated that "lamination is produced by continuous or uninter­rupted sedimentation which resembles the lamination generally assumed to be produced by seasonal deposition". The "varves'' therefore may not have any particular chronological significance, or at most only a very local one.

t HADDINC, AssAR, The First Rains and their Geological Significance: Smith­

sonian Report for 1930, pp. 285-294, 1931.

2 SEDERHOLM, J. J., Pre-Cambrian of Fennoscandia: Bull. Geol. Soc. Am., v. 38,

pp. 824, 826, 1927.

3 K INDLE , E. M., lnfluence of Temperature on Color and Lamination in Sedi­

ments: Report of the Committee on Sedimentation, National Research Council,

p. 42, 1924.

GEOLOG Y OF THE MT. GAUSTA REGION IN TELEMARK 29

The grains are sorted so that the coarser materials Iie at the south side of the steeply dipping or alm ost vertical beds: the south

side was then presumably the bottom of the formation as a whole. The rocks have been steeply folded, but not overturned, in the region south of Mt. Gausta; unfortunately no specimens from Rjukan were marked in the field for comparison on this point.

Once deposited, the sediments passed through a complicated series of metasomatic changes. The first of these changes may have taken place comparatively soon after deposition, when hydrothermal solutions, connected perhaps with the same cycle of igneous activity, caused the albitization and sericitization not only of the tuffs but also of the rhyolites and vitrophyres associated with them. Then followed a period of dynamic metarnorphism, with deformation or obliteration of the original textures, and with some recrystallization producing a parallel texture, especially in the more sericitic portions of the rock. During the final stages of this period, hydrothermal action was renewed, introducing iron ores, biotite, zircon and quartz. Later movements produced cracks through which hydrothermal solutions aga in percolated: chlorite was introduced, in places metasomatically replacing bi o tite; iron oxides, quarts, and finally calcite are seen as vein fillings from this period.

These two later periods of hydrothermal activity are undoubtedly connected with igneous intrusions. The intrusives corresponding to the first period are difficult to identify: possibly some of the highly altered basic rocks occurring as dykes among the volcanic rocks are evidences of this igneous cycle. The second period was without doubt connected with the numerous masses of gabbro in the region. These were intruded after the upper series of sediments -now quartzite, arkose and slate - was Iaid down, and will be dis­cussed in a later section.

T h e L a v a s.

Subdivision into types: Associated with the tuffs, and far more extensive in the field, is a series of volcanic Javas. These are por­phyritic or nearly non-porphyritic, massive or somewhat schistose in texture, light or dark in color (though microscopic examination reveals that even the dark ones are of acidic composition). The separation of one type from another in the field is not always easy, for clear-cut

30 DOROTHY WYCKOFF

contacts rarely occur. Map 4 gives the areal distribution of the types he re distinguished:

Type l. Dark colored massive Javas, non-porphyritic or with a few minute phenocrysts of feldspar. Flow structures are commonly seen, lithophysæ in a few places.

Type 2. Dark colored porphyry with numerous small phenocrysts (up to 5 mm.) of feldspar (rarely quartz). Contains numerous inclusions.

Type 3. Light grey massive porphyry with large phenocrysts ( 1.5�2 cm.) of feldspar and quartz.

Type 4. Light grey-green massive or schistose porphyry with phenocrysts (0.5�1 cm. in size) of quartz and feldspar. Large lithophysæ and flow breccias are found in many parts.

Type 5. Non-porphyritic dense "flinty" Javas, light yellow, green, pink or purple in color. This may be only a non-porphyritic facies

of Type 4. Description of Types: Type 1: This is a dense, dark colored

rock, ha ving a rather massive character; diagonal joint plan es are clean, flat and rather widely spaced. One prominent, almost vertical set, running in general northwest�southeast, happening to coincide with the general direction of ice-movement in the region, has favored the production of small ridges and valleys in places where this rock is exposed. This is well seen in lower Tveitåodal and near Tuddal, but not in the area shown on Maps 3 and 4. Weathered surfaces display sinuous flow iines, and what appear to be strings or clusters of lighter colored spherulites (Photograph 8). The fracture is con­choidal, and the texture so dense that even with a hand lens little or nothing of the mineral composition can be made out. Phenocrysts are rare and very min ute: in many cases they were discovered only in the thin section of a rock supposed in the field to be non-porphyritic.

Under the microscope this rock shows evidences of a his tory similar to that of the tuffs. Specimens of the more massive uniform material are seen to be made up of a minutely fine-grained aggregate 1 of anhedral quarts, albite, microcline and sericite. The most striking

l In dealing with all !hese very dense rocks, determinations of the constituent

minerals in thin section were checked against determinations made by the

immersion method on crushed material. The use of a liquid with n � 1.525 made

possible the determination of the presence or absence of microcline and al bite,

and a rough estimate of the proportion of each, as well as that of the quartz,

in the groundmass.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 3 1

thing about this aggregate i s its "patchy" or "mottled" extinction,

which is caused by optical continuity of one of the minerals (com­

monly the quartz, where the grain is not too fine for the identification

of the materials) , in which Iie unoriented grains of the other minerals. This seems to be the texture called micropoikilitic by WILLIAMS 1.

I ts exact nature and pro bable origin are more easily discovered in a

series of slides where the flow structures, so striking in the field,. were sectioned for study.

Phot. 8. Exposure of Type l porphyry, showing flow lines and joints.

(Length of hammer 65 cm.).

Such sections reveal an immense amount of metasomatism. Thin Jayers of fairly uniform material having the micropoikilitic texture, and containing a good deal of very fine sericite, alternate with bands where the crystallization is coarser, and the relations of the minerals more distinct. The bands have the appearance of fine veins filled with hydrothermal minerals, in a typical "comb-structure". Implanted

on the walls are radiating crystals of microcline, projecting inward toward the center of the "vein". The microcline is partially or al most com­pl ete! y replaced by albits. The two minerals are readily distinguishable

by their indices of refraction, and in some places by their twinning,

I WILLIAMS, G. H., On the use of the terms Poikiiitic and Micropoikilitic in

Petrography: jour. Geo!., v. I, p p. 176-179, 1883.

32 DOROTHY WYCKOFF

though much of the albite is untwinned. Between the feldspar laths, and filling the center of the "vein" where it is wider, is clear gra­nular quartz. The extinction of the "veins" is in sections, since the quartz which includes a number of radiating groups of feldspar crystals is in optical continuity. Where such aggregates are cut tan­gentially, they differ only in their greater size and coarser texture from the "patches" of the groundmass; and in some places a complete gradation from coarser to finer patches may be traced.

This texture is characteristic of man y ancient Javas 1• It has been thought to be due to devitrification 2, but in the Mt. Gausta region, at !east, it seems to be the result of metasomatic replacement.

Associated with the quartz in the center of "veins" is a small amount of strongly pleochroic brown biotite, similar to that found in the tuffs. Here too it seems to have been introduced at a compara­tively late epoch in the rock's history, for most of it is found along a network of intersecting cracks which are in places roughly parallel to, or even follow, the quartz-feldspar "veins", but in other places cut across them nearly at right angles. In the latter circumstances, it is apparent that the quartz-biotite "veins" (with some anhedral magnetite and titanite) are later, and that no albite was associated with that phase of hydrothermal activity. Only one slide shows calcite and chlorite with the biotite; the chlorite has characters similar to those of the chlorite (aphrosiderite) found in the tuffs.

The rare phenocrysts are of al bite (Photograph 9), or of microcline showing extensive replacement by albite.

The rock has undergone a bodily replacement - metasomatism in the fullest sense of the word - and it may be doubted whether any of the original minerals remain. Yet on first examining a thin section by ordinary light with a low power objective, one is struck by the distinct pattern of flow lines, winding gently around the phenocrysts and inclusions, and by small round bodies like minute

' BASCOM, F., The Ancient Volcanic Rocks of South Mountain, Pa.: Bull. U.S.G. S.

136, 1896. This paper gives references to earlier descriptions of rocks of this

type, of which NoRDENSKJOLD's account of the Swedish "ha!leflinta" is of

particular interest for comparison with the rocks of the Mt. Gausta region.

NORDEN SKJOLD, OTTO, Zur Kentniss der sogen. Ha!leflinta des nordost­

lichen Smålands: Bull. Geo!. Inst. Upsala, bd. l, 1893.

2 This is the point of view of o ne of the most recent pa pers on such rocks:

ST ARK, J. T., Igneous Rocks in the Baraboo District, Wisconsin: Journ. Geo!.,

v. 40, pp. 119--139, 1932. Exce!lent photomicrograph on p. 129.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 33

Phot. 9. Thin section of Type l porphyry, showing phenocrysts of albite.

(Crossed nicols).

Norsk geo!. tidsskr. XIII. 3

34 DOROTHY WYCKOFF

spherulites, all of which are completely masked by the secondary crystallization when the analyser is inserted. The greatest albite­quartz replacement has taken place along certain of these flow bands; and in one thin section, where the spherulites seen by or­dinary light are exceptionally large, they are found to be made up of a radiating mass of feathers of albite, with a wavy extinction approximating the cross seen in true spherulites. The center is filled with larger crystals of al bite and quartz; and the whole spherulite is somewhat richer in sericite than adjacent areas. This difference in sericite content is also seen in adjacent flow bands, and it seems to be solely on this account that such structure are visible by or­dinary light.

In some places lithophysæ occur. WERENSKIOLD l noted them at Solem farm, about 2 km. south of the southern boundary of Map 4; but they are locally abundant in many other places. They are small (up to 3-4 cm. in size), usually somewhat flattened, and in the hand specimen appear to be made up of thin concentric shells of white and greyish material, whith a center ( 1-2 cm. in diameter) of greyish quartz.

Under the microscope a section shows layers of pure granular quartz alternating with layers made up of a much finer granular aggregate of anhedral albite, sericite, and a little quarts. The center

is not visible in this section. The outermost shell is much thicker than the others, and composed of coarser granular quartz which is continuous with several small branching veins running out into the groundmass. The groundmass is similar to that of the lava elsewhere, though in this case the sericite has a parallel orientation suggesting pressure schistosity, and there is a much more extensive introduction of quartz, with a little biotite.

The history of this rock is similar to that outlined for the tuffs. The lava must have solidified as a glass, containing a few small phenocrysts of potash feldspar, as well as spherulites and lithophysæ in some parts. Possibly complete devitrification took place, but the earliest change which can now be traced is the extreme albitization, affecting not only the phenocrysts but also all those finer structures, spherulitic or lithophysal, which were made up presumably of potash

l WERENSKIOLD, WERNER, Om Øst-Telemarken: N. G. U., nr. 53, Årbok for 1909, p. 37, 1910.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 35

feldspar. The released potash was used up in the formation of the

sericite, which by its present distribution bears witness to original

differences of composition in different parts of the lava. Extensive

quartz-replacement accompanied and followed the albitization; at this time developed the micropoikilitic groundmass, with its patches of

quartz including smaller unorientated particles of albite and sericite. Subsequently a period of dynamic metamorphism produced a slight

schistosity in some places, and opened cracks through which another

Phot. 10. Exposure of Type 2 porphyry, showing inclusions.

{Length of hammer 65 cm.l.

percolation of hydrothermal solutions took place: iron o res, biotite, titanite, and more quartz were introduced. A still later period of hydrothermal activity, with the introduction of chlorite, epidote, quartz and calcite is only faintly indicated by this suite of thin sections, but it can be traced in other Javas not far distant.

Type 2: In the field this lava is readily distinguished from

Type l by the presence of numerous, though small (about 5 mm.)

phenocrysts of plagioclase feldspar and (subordinate) quartz. There is a greater tendency to schistosity in many places. Flow structures are less commonly seen, and spherulitic and lithophysal crystallization

appear to be lacking, at !east within this area. Inclusions are locally

very numerous (Photograph l 0).

36 DOROTHY WYCKOFF

Under the microscope, the most interesting features shown by this series of specimens are the phenocrysts, the numerous inclusions, and the extreme biotitization.

The phenocrysts, both quartz and feldspar, show rounding and embayments caused by magmatic resorption. The feldspars exhibit all stages of the replacement of original potash feldspar (now micro­cline) by albite. It is of interest to note that this albite is thickly filled with sericite, while adjacent areas of microcline are clear: this indicates the formation of sericite as a "by-product" in the albitization process. The microcline has vague "grid" twinning, while the albite twinning is as a rule very fine and clear, with a typical "chess-board" pattern. In some slides the phenocrysts are strained or broken by pressure, and the cracks are commonly filled with quartz, which has in a few cases partially replaced albite.

The inclusions found in this lava range from a microscopic size up to 10- 15 cm. in length. On weathered surfaces they are commonly darker than the matrix (Photograph l 0). Under the microscope the y are seen to be of various compositions and textures (Photograph 1 1 ) .

A few have the typical micropoikilitic texture of Type l. Others are made up almost entirely of quartz in a fine mosaic, with a little sericite in the interstices. Still others are composed almost entirely of long, thin laths of al bite, with a little sericite; these con ta in more or less biotite, and all gradations are found between these and frag­ments composed almost entirely of biotite, with a little interstitial quartz, sericite, and albite (and such accessories as iron oxides and rutile). The varied character of these fragments is perhaps an argu­ment, though not a very strong one, against the hypothesis that all are cognate inclusions.

The whole rock has undergone intense biotitization, affecting different inclusions to a different extent. Those composed chiefly of quartz show as a rule a shell of biotite wrapped around them, and little or none in the interior. The more feldspathic inclusions show biotitization along the edges or in patches in the interior; and in others, practically all the original material has been replaced by biotite.

The groundmass as well has been thoroughly altered, and if it ever had the micropoikilitic texture, this has been completely obscured by recrystallization in which the chief minerals were quartz and biotite. The finest part of the groundmass is made up of quartz,

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 37

albite, and abundant sericite, in a microgranular texture. The seri­

cite appears to have been more or less orientated by pressure, but in one case there are distinct flow lines, rich in sericite, winding about the phenocrysts and inclusions. This occurs in a thin section where the phenocrysts are remarkably clear and unstrained, and there

can be no doubt that it is an original structure. Later veins contain iron ores, quartz and calcite.

Whether the intense biotitization of the Type 2 porphyries is due to local circumstances (e. g., the numerous greenstone dykes which occur within the area), or whether it indicates an original magma richer in such elements as iron and magnesia, so that biotite was more readily formed, is a question not easy to decide.

Rocks of a very similar type are found in Vestfjorddal at Rjukan, but in them biotite is almost or entirely lacking. Whether they really belong to the same flow as the rocks here designated as Type 2 is more than doubtful; but examination of them gives some information about the changes, apart from biotitization, which may have occurred in rocks of this type.

The dominant alteration in this case is albite- and quartz-replace­ment. The scanty biotite seen in one thin section is evidently of later introduction; it is partially chloritized.

The phenocrysts are similar to those al ready described: they show small patches of microcline in the midst of "chess-board" albite, which is in places penetrated by clear quartz; the latter has not uncommonly a peculiar dendritic pattern, approaching a myrmekitic intergrowth with albite.

The groundmass is extremely fine-grained. With a highpower objective parts of it can be resolved into minute grains of a mineral with low index (n < 1.525) and very faint birefringence. This is probably potash feldspar, derived by devitrification from the original glassy base of the rock. In this material are found grains of albite and quartz. The whole process of replacement by these two minerals can be traced, through the formation of the micropoikilitic texture, to a highly silicified rock, in which veins and patches of secondary quartz make up most of the groundmass.

The inclusions found in this rock are conspicuous in ordinary light by reason of the abundant fine grains of magnetite uniformly distributed through them; otherwise they differ in no respect from their matrix. They are almost certainly cognate inclusions.

38 DOROTHY WYCKOFF

Cataclasis is noticeable in all these thin sections. The pheno­crysts are strained or fractured, and the sericite in the groundmass is arranged along shear-planes, imparting an incipient schistosity to the rock.

In view of the similar replacement-history seen in the pheno­crysts of both groups of rocks, it seems not unreasonable to conclude that the groundmass of rocks of Type 2 has also passed through the stages of devitrification, albitization and silicification, which the later phase of quartz-biotite crystallization has almost or quite ob­literated.

Type 3: This is a porphyritic rock of a totally different aspect from the preceding types. The groundmass is dense, light grey in color, with small flakes of biotite, especially along cracks. The con­spicuous phenocrysts of quartz and feldspar are large (I .5--2 cm.) but smaller phenocrysts are also present. The character of the rock is massive, and no regular system of joints is discernible.

Under the microscope, two generations of phenocrysts are seen: small ones, about the size of those in Types l and 2 (0.5 cm. or less), and very large ones ( 1.5-2 cm.). All are resorbed and embayed, and have suffered both hydrothermal alteration and mechanical deformation. The large ones are extensively granulated: the quartz shows wavy extinction, and the twinning lamellae of the feldspars

are curved and faulted. The microcline has been almost entirely replaced by "chess-board" al bite filled with a fine "dust" of sericite; quartz replacement has in places followed the albite replacement.

The groundmass is made up of spherulitic structures, showing between crossed nicols black crosses which open into hyperbolae as the stage is revolved. With high magnification these spherulites are resolved in to an al most submicroscopic intergrowth of two minerals: quartz, and hairlike, curved rods which have negative elongation and an index of refraction corresponding to potash feldspar. The quartz fills the interstices between these rods, and is more abundant in the outer part of the spherulite. Tangential sections of such spheres show a fine granophyric intergrowth of quartz and feldspar. Sericite is present throughout the whole groundmass. ( Photograph 12).

Scattered about among these spherulites Iie tiny skeleton crystals of feldspar. Their index of refraction indicates albite, but the patchy character of the albite suggests a replacement of potash feldspar. The delicate forked ends and hollow centers enclosing part of the

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 39

Phot. Il. Thin section of Type 2 porphyry, showing resorbed phenocrysts

and inclusions. (Piane polarized light).

40 DOROTHY WYCKOFF

groundmass are typical of skeleton crystals found in modern glassy Iavas, and could not possibly be forms of feldspars crystallized after the rock had solidified.

Other portions of the groundmass show a coarser crystallization of quartz and albite (replacing microcline) like the "veins" described in rocks of Type l. Here, however, there are no continuous veins, but only irregular patches, indicating areas of complete replacement of the original groundmass.

A network of later cracks traversing both groundmass and pheno­crysts is filled with biotite (Iocally collected into little "nests"), quartz, apatite, magnetite, titanite, and also chlorite and calcite. The chlorite (aphrosiderite) has replaced some of the biotite.

This rock seems to have had a complex history, even before its first solidification: four stages of incipient crystallization are in­dicated by

1. The large phenocrysts \ . 11 f partta v 2. The small phenocrysts ·

3. The skeleton feldspars 4. The spherulites, indicating a final

ing soon after 3. Whether there was any glassy base

resorbed

rapid crystallization, follow-

is uncertain.

After solidification, the mineral sequence indicates a series of alterations similar to that seen in the other volcanics: albitization, sericitization and extensive quartz-replacement, followed by a period of dynamic metamorphism, when the phenocrysts were sheared and granulated, and the spherulitic groundrnass deformed. Minerals of the later sequences - biotite-quartz, and chlorite-quartz-calcite - are also present, though in small amounts.

Type 4: This rock is lighter colored than any of the preceding types, and is distinguished in the field by its massive character, though locally it may be quite schistose, with much greenish-silvery sericite. The phenocrysts, of quartz and feldspar, are distinct and numerous, though not large (average about 0.5-1 cm.). Lithophysæ, flow lines. and flow breccias are in places well developed. They are most striking on weathered surfaces, which are greenish-white, yel­Iowish or stained with pink.

Microscopic study reveals in general the same type of alteration as in the other Iavas. Quartz replacement is, however, more extensive

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 41

Phot. 12. Thin section of Type 3 porphyry, showing spherulitic groundmass.

(Crossed nicolsl.

42 DOROTHY WYCKOFF

than albite replacement, and in three thin sections from one locality no albite is discoverable: the mlcrocline phenocrysts are in process

of alteration directly to sericite, and are partially replaced by quartz.

When far advanced, this type of alteration leaves only a patch of fine, scaly sericite, with some interstitial quartz, in place of a phenocryst.

The same process'go'es on also in the groundmass, which is in these

thin sections composed essentially of quartz and sericite, with a little microcline. Commonly, however, albitization can be traced, though

Phot. 13. Exposure of Type 4 porphyry, filled with lithoph�sae.

(Length of hammer 65 cm.).

to a less degree than in the other Javas. A great amount of quartz has been introduced; the micropoikilitic tex ture is in places developed;

and more advanced silicification gives a groundmass traversed by a network of veins and patches of quartz. If biotite and iron oxides

are present they are, as usual, associated with this vein quartz.

The introduction of biotite, chlorite, and other later minerals

may be studied to advantage in thin sections where these minerals are abundant. The most extreme example of biotitization is found in a specimen which was collected as an "intrusive basic rock (?)".

Microscopic study, however, reveals a groundmass essentially similar

to the nearby porphyries: it is a fine-grained aggregate of sericite

and quartz, with a very little albite, and shows here and there a

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 43

micropoikilitic texture. The whole thin section is filled with large

porphyroblasts of brown biotite. These have unusually large, dark

pleochroic haloes, and are slightly alte red to chlorite; they are ac­companied by iron oxides and quartz. Calcite appears here, as

elsewhere, to have been introduced latest of all. This specimen has

undergone intense hydro­

thermal alteration, probably

in a contact zone, though at

this particular outcrop the

intrusive rock itself either

did not reach the surface, or

was overlooked because of its

resemblance to the biotitic

rock of the contact zone.

Another specimen, col­lected where several small

greenstone dykes from the

gabbro cut the porphyry,

shows a groundmass (which

has a faintly micropoikilitic texture) completely pene­

trated by anastomosing veins and stringers of quartz. The biotite is similar in

habit to that in the prece­ding specimen, but is Jargely altered to a bright green chlorite (aphrosiderite). Cal­cite is abundant; and a few small veins cutting across Phot. 14. Thin section of lithophysa from Type 4

porphyry. (?!ane polarized light). the general network, and bearing only quartz, chlo-rite (in small scales, without biotite), and calcite, indicate that these

minerals belong to a later period than the biotite. Associated with these later veins are a few tiny zircons, and large euhedral crystals of magnetite (in distinct, shining octohedra, up to 0.75 or nearly

l cm. in size) and pyrite (in typically striated brass-yellow cubes, of which the !argest found measured over 2 cm.). These large

.crystals

rarely include any of the groundmass material, but may be cleanly

44 DOROTHY WYCKOFF

detached from the rock, since they are surrounded by a sheath of chlorite, in plates parallel to the crystal faces. The connection of these later minerals with the nearby dykes and sheets of greenstone seems certain in this case.

The lithophysae found in this lava are larger and more perfect than those previously described. On weathered ledges great masses of them are exposed; the whole rock then seems to be made up of white spheroids, ranging in size from 1-2 up to 15-20 cm., in a scanty, schistose groundmass containing much silvery-green mica (Photograph 13). The spheroids are readily detached from the ground­mass, and in some cases one or two thin shells can be spalled off the more weathered side of the spheroid. It is said that specimens from a locality a few km. farther east, which are now in the Geological Museum in Oslo, were sent in as "fossil oysters".

Under the rnicroscope a small spheroid of this sort shows a center of quartz in interlocking grains, merging outward into a zone of fine concentric bands (Photograph 14). Some of these bands are composed of anhedral, finely granular quartz; alternating with these are bands of feldspar. This mineral is completely anhedral: the index is distinctly less than quartz, and usually lower than, sornetimes nearly equal to, balsam: it is pro ba bly potash feldspar, with some albite. Colorless sericite, in minute shreds, is abundant. These

feldspar bands are in general narrower and made up of smaller grains than the quartz bands; the outline between the two is irregular, and bands pinch out or unite.

The groundmass between the lithophysae is similar to other parts of the porphyry. Albite replacement of the microcline phenocrysts is seen in a small scale, while quartz replacement is extensive to alrnost complete. Some of the quartz phenocrysts are euhedral, terminated by pyramidal faces, but most are (like the feldspars) much resorbed; secondary enlargernent, by the growth of quartz in optical continuity with the phenocryst, is not uncomrnon. The ground­mass is rich in sericite, with an arrangement suggesting an originial flow structure, but this may have been modified by pressure, for many of the phenocrysts, as well as the lithophysae, are cracked and traversed by later quartz veins. Tourmaline, zircon, and titaniferous magnetite or ilrnenite extensively altered to leucoxene, are found in these veins.

The lithophysae are surrounded by an incomplete fringe of tourmaline. The crystals Iie in radiating groups projecting into the

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 45

groundmass; in places the y stand al most normal to the spheroid's

surface, as in orbicular or spherulitic structures. Their pleochroism is strong: O blue green, E yellow to colorless, O> E.

Probably these minerals were deposited by solutions connected with the granites found in Gausdal, though this exposure is at !east 1.5 km from the nearest known outcrop of granitic material.

Type 5: This is perhaps hardly a separate type: the tracing of its contact with Type 4 was attempted in the field, but there is no abrupt transition from one to the other; it may be only a non­porphyritic facies of Type 4. Certain outcrops, however, have a very different appearance from the other Javas. They are of a pale yel­lowish, pink or purple color, with a peculiar dense "flinty" aspect, and a slightly schistose structure, though little or no sericite can be detected in the hand specimen.

U oder the microscope they display no unusual features. The phenocrysts are extremely small and few, but otherwise similar to those of Type 4. The groundmass shows a perfect development of the micropoikilitic texture, with sericite in small scales; in two cases it is somewhat flattened by pressure. Later veins, with scanty biotite or chlorite, occur, but are rare. Iron oxide, as a fine dust uniformly disseminated through the entire slide, seems to be an original constituent of o ne specimen; this by weathering un­doubtedly gives rise to the pink or purple colors of some outcrops.

The Flow Breccias: Extensive flow breccias are found in la vas of Types 4 and 5. The fragments included in these breccias are of many different kinds. One thin section contains many angular bits

like shards of glass, of which some show a micropoikilitic texture, some are completely albitized like the glass fragments in the tuffs, and some are filled with small, closely crowded spherulites showing clearcut black crosses in polarized light. Another thin section con­tains fragments made up of granular quartz and sericite, or of quartz. al bite and sericite; and also similar fragments thickly dusted with black iron oxide. Still another has fragments with a fine sericitic groundmass and small rounded phenocrysts of microcline; and similar, but non-porphyritic, fragments preserving beautiful perlitic cracks. Albite and quartz replacement are plainly displayed, and minerals of later introduction - biotite, chlorite (aphrosiderite ), zircon, quartz and calcite- also occur. Tourmaline is found in one specimen from lower Gausdal.

46 DOROTHY WYCKOFF

The most interesting exposure of a breccia is in Gausdal, about km. north of Reisjåvatn (Photograph 15).. Here are seen, on

weathered surfaces, angular fragments of all sizes from a few cm. up to several meters in length. On the trail is exposed a ledge of steeply dipping, thin bedding material, which appears very similar to some exposures of the bedded tuffs. It is a dense light grey rock which separates readily in to la y as (0.5 -4 or 5 cm. thick), and on the surfaces of these layers are beautifully preserved ripple-marks. The

Phot. 15. Exposure of ripple-marked sediment in volcanic · flow breccia, Gausdal.

(Length of hammer 65 cm.).

assymetric "wave" form of these ripples, the criss-cross direction of two sets observed on a single surface, and the differing amplitudes of the ripples on two sides of a thin slab (Photograph 16), are con­clusive evidence against any hypothesis that such markings are in this case produced by dynamic metamorphism: they do not in the !east resemble any sort of pressure-cleavage or schistosity.

Under the microscope this material is seen to be a very fine­grained aggregate of sericite, quartz and albite. The sericite has a sub-parallel arrangement, lying at an angle of about 25" to the bedding. Large porphyroblastic crystals of biotite, accompanied by quartz and iron oxides, and partially altered to chlorite (aphrosiderite) , are present.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 47

The whole appearance of the rock is in many respects very similar to that of the tuffs. In any case, it is certainly a sedimentary rock, though its present position, isolated within a rather large area of flow breccias, gives no indication of its origin. It is certainly extra­ordinary that the ripple-marks should be so perfectly preserved -"fossilized", as it were -by being embedded in a volcanic lava.

Interpretation and history of the volcanics: If the fragment of sedimentary rock described in the preceding section be considered

Phot. 6. Hand specimen from exposure shown in Phot. 15. The slab is l cm. thick and has ripple-marks of different amplitudes on the two sides.

(Scale 15 cm. long).

a fragment of the bedded tuffs which it so strongly resembles, the obvious conclusion is that the tuffs were formed and already some­what consolidated, before the time when this lava was poured forth. While this one bit of evidence is very insecure, the conclusion is borne out by the observed contacts between the tuffs and the ad­jacent porphyries, especially the porphyries of Types l and 2. Along the contact with Type 2 the bands of the tuffs are often disturbed, bent or minutely faulted; and in the brook which for about a kilo­meter follows on or near the contact, many ledges are exposed in which the porphyry can be seen injecting the tuffs in a small scale. Moreover, small areas of the tuffs appear within the porphyry, and

48 DOROTHY WYCKOFF

small areas of the porphyry within the tuffs: the map shows on ly the areas within which one or the other predominates. The same thing is seen, though at only one or two places, on the much shorter contact of the tuffs with Type l.

The contact of the tuffs with the Type 4 porphyry has been large) y inferred fr m the topography: Bua rås and Reisjåfjell are composed of rather massive porphyry; and under the scree at the foot of these steep or cliffy ridges the tuffs appear. Farther to the northeast, where the topography is milder, considerable stretches are covered by bog and heather. However, almost every exposure of breccias found in Javas of Types 4 and 5 shows more or less weathered fragments of a delicately band ed rock; the bands are nearly always straight - a characteristic which would hardly be so constant in fragments derived from a rhyolite with flow banding. This fact makes the hypothesis that the large ripple-marked fragment is also a portion of the tuffs seem less improbable.

The question of the relative ages of the porphyries is not at all easy to decide. It was thought from field evidence that the numerous dark inclusions found in the Type 2 porphyry along its contact with Type l might have been derived from the latter. Microscopic evidence does not belie this interpretation: some of the fragments are very similar in appearance to Type l ; but fragments of other kinds are

likewise present; and cognate inclusions might also be expected in a lava. The same dubiousness attaches to evidence from the character of inclusions in the flow breccias of Types 4 and 5.

Type 3 presents a different problem. The main exposures of this porphyry are along a straight line running nearly north and south, between Reisjåvatn and Kovstulvatn; two other exposures west of Reisjåvatn are also on a north-south line. This indicates a cross­cutting relationship of some sort, though the presence of exposures of other porphyries (and tuffs) between exposures of Type 3 indicates that the latter are not parts of a continuous sheet or dyke. In spite of prolonged search, no actual contacts were disco ve red: the Type 3

phorpyry along the major line of exposures occurs mostly on the tops of rounded hillocks or roches moutonnees, surrounded by little

depressions (many of them too slight to be noted on the topographic map), which are filled with sphagnum bog, heather, or glacial gravels. The locality west of Reisjåvatn is one with many small intrusive dykes from the gabbro, and all the porphyries are much altered.

GEOLOGY OF TH E MT. GAUSTA REGION IN TELEMARK 49

There can, however, be no doubt that all these volcanics are practically contemporaneous, belonging to the same igneous cycle. An earlier period of explosive activity seems to have been followed,

in this region at !east, by a period of quieter emission of lava sheets or flows. The intrusive contacts found represent the cross-cutting of one lava flow through another, or through a bed of ash, during eruption. The Type 3 porphyry may have filled fissures or vents through which Javas reached higher levels.

Some evidence of the original attitude of the volcanic series is afforded by the sorting of the particles in the fine layers of the bedded tuffs, with the coarser material at the south side of each layer. The whole formation is now steeply dipping toward the northwest in the region south of Mt. Gausta; while similar rocks, both tuffs and Javas, north of Mt. Gausta in Vestfjorddal, dip steeply southeast or south. It is possible, though by no means proved, that the whole is one structure - a steeply folded syncline. But it is at !east equally possible that members of similar character, tuffs or Javas, may recur several times in a volcanic series, and that more complicated folding and faulting accounts for the varying dips in dif­ferent places. The greenstone tuffs and coarse agglomerates, found at Rjukanfoss in close proximity to thick beds of finely banded tuffs, seem to have no equivalent in the region south of Mt. Gausta.

The alterations undergone by the whole volcanic series seem to indicate the following sequence of events:

l. Hydrothermal activity, resulting in a. devitrification of vitric tuffs and glassy Javas, b. albitization of glass and potash feldspars, c. formation of sericite at the expense of potash-bearing minerals

or glass, d. extensive quartz-replacement of earlier minerals, especially al bite,

2. Dynamic metamorphism (folding and faulting (?) of the whole series) perhaps accompanied by the intrusion of basic rocks (now highly altered biotite schists).

3. Renewed hydrothermal activity, with introduction of much biotite, and quartz and iron oxides.

4. Milder dynamic metamorphism, accompanied by the intrusion of the granites and gabbros of the region, with formation of new cracks and joint planes.

Norsk geo!. tidsskr. XIII. 4

50 DOROTHY WYCKOFF

5. Still another period of hydrothermal activity, with solutions con­nected with the granite and gabbro magmas introducing tour­maline, epidote, chlorite, quartz and calcite. "Ore" minerals such as magnetite and pyrite are also traceable to this period.

The Sediments.

Description: Overlying the volcanics is a series of sediments -arkose, quartzite and slate - making up all of Mt. Gausta, the ridges southwest of Gausdal, including Bonsnås, and also the top of Hedders­fjell. The lowest beds are in general the most feldspathic, and in places contain pebbles or angular fragments up to 2-3 cm. in size. The ar kose is light pinkish to yellowish grey in color, weathering lighter; or greenish where the feldspar is altered to epidote. Higher up, beds of arkose and quartzite alternate, the sumits of all the mountains and ridges being composed of resistant quartzite (Photograph 17). This quartzite is white to violet grey in color, made up largely or entirely of quartz. The beds are massive and the rock weathers in huge cuboidal blocks (up to 2 m. in size), which have in places been "heaved" by frost. Ripple-marks are visible on some bedding planes. On the southwest side of Bonsnås there are clay-slates overlying the quartzite, and very thin layers of similar materials are interbedded with the arkose on the northeast side of Mt. Gausta. The slate is dull blue-grey in color; the layers are thin and irregular, with a pronounced fissility.

Under the microscope the pure quartzite exhibits a typical tex ture: rounded grains, showing secondary enlargement, in a closely interlocked mosaic. The interstitial material - quartz and fine, scaly sericite - is relatively small in amount.

The various arkosic layers show varying proportions of materials other than quartz. Microline and albite feldspar are common. Frag­ments of earlier rocks are found in almost every thin section, being in some the predominant constituent of the rock as a whole.

These rock fragments are of several different kinds. Most easily recognizable are fragments strongly resembling the volcanic rocks previously described. Many of these show unmistakeably the micro­poikilitic tex ture; more rare! y they show a band ed structure, though not well enough preserved to make it possible to decide whether it is like that of the banded tuffs, or a flow structure of some sort.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 51

Sericite is abundant in some of these fragments, and may show a distinct parallelism, as if the rock had been subjected to pressure. All these fragments seem to be derived from acidic volcanic rocks. The angularity and fragility of some of the larger ones indicates that the y have not been transported far; but man y are well rounded. Less numerous, but of considerable amount in some thin sections, are fragments which appear to be much altered basicic rocks. In a

· few cases the fragment shows traces of an original ophitic texture,

Phot. 17. Bonsnås from the south, showing massive beds of quartzite

dipping southwest.

the mafic minerals being replaced by black iron oxides. In most cases, however, the feldspars are completely saussuritized, and the fragment is simply a fine-grained aggregate of quartz, albite, zoisite (or epidote), and iron oxides. There are also accessory minerals - titanite and zircon - which are rounded and appear to have accumulated with the sand from which the arkose is formed.

A rough count was made on ten thin sections, and the results are tabulated below. Five fields of view were taken in each section, and a co unt made of all the grains: the num ber of grains of quartz, of feldspar (microcline and albite), of rock fragments. (acidic and basic), and of accessory minerals, was thus determined. The averages of results are not given as percentages of the rock composition be-

52 DOROTHY WYCKOFF

cause the count disregarded the interstitial material (mostly sericite and quartz) , which is considerable in amount in certain thin sections, especially where the rock has been subjected to press u re; and also disregarded minerals introduced by hydrothermal solutions (chlorite, biotite, epidote, calcite, etc.) which are abundant in specimens taken near the contacts of igneous intrusives. The result, however, gives an approximate idea of the proportions of the various constituents:

No. grains Micro- Acidic Basic Acces-

No. in field Quartz el i ne

Al bite rocks rocks sories

of view

l 48 l 20 2 4 22 - -2 36 14 6 3 13 a -3 24 13 - 5 6b a a 4 28 13 3 2 8 2 -5 54 9 l 6 30 8 a 6 47 8 5 3 26 5 -7 62 20 5 10 24 3 -8 40 40 - - a a -9 28 19 6 l 2 a

l -

lO 63 63 - - a a -

a - constituent present in appreciable amount, bul less than one grain to each

field of view.

b- this thin section has also large (0_5-0.75 cm.) fragments which were avoided

in making the count.

The specimens are arranged in the table roughly in order of their distance from the base of the sedimentary series: no. 1-5 are ne ar the lower contact at various points; no. 6 is a litt le high er up; no. 7, 8, 9, and 10 are about 200, 300, 450 and 550 m. respectively above the lower contact. The count confirms the field impression that the lower beds are composed of more mixed materials, with rock fragments etc., while the upper beds are more nearly pure quartz. But no. 9, high up on Mt. Gausta, contains a notable amount of feldspar and a few rock fragments; and the re are thin beds of almost pure quartzite lower down in the series not represented here. The gradation is by no means even or continuous. Nor do the grain sizes vary continuously from coarser at the bottom to finer at the top. No beds containing very large fragments have been found except at the bottom of the series; but no. l is finer than several beds which Iie above it; while just above no. 7 Iies a thin bed of clayslate, with coarser beds like no. 9 still higher up.

GEOLOGY OF TH E MT. GAUSTA REGION IN TELEMARK 53

The quartzite forming the higher part of Mt. Gausta, dipping 30° southwest, forms also the top of Bonsnås and the other ridges southwest of Gausdal, and on the southwest side of Bonsnås is overlain by slate. Under the microscope a thin section of this slate is seen to be made up of irregular coarser and finer layers; the coarser layers contain some small angular grains identifiable as quartz, and a much lesser amount of microcline. Abundant sericite is present in fine scales, most of the rock being composed of a minutely fine­grained aggregate of sericite and quartz.

The textures of the rocks give evidence of varying amounts of pressure. The pure quartzite seems to be most resistant to crushing. The quartz grains show slightly wavy extinctions (which may be in part inherited from a period of dynamic metamorphism of the rock from which they were deri ved); but with increasing pressure there is cracking of the grains, with a greater development of interstitial materal, and a tendency to parallelism of the sericite flakes.

The more arkosic beds show greater evidence of pressure. The quartz and feldspars show wavy extinctions (though again this may be in part due to an earlier dynamic cycle); but there are also cracks, shearing of broken grains, and traces of marginal grinding.

Sericite is developed in greater amount and shows a greater tendency to parallelism, which may be more or less uniform throughout the rock, or may be greatest along small shear planes, imparting an incipient schistosity to the rock as a whole. The rock fragments, when present, se em to be rather resistant to crushing; and in cases where the fragments contain sericite, the orientation of the sericite within the grain is often quite different from that of the sericite in other grains or in the interstices between grains.

The clay-slates show a pronounced parallelism of the fine seri­cite which is a chief constituent. It is a question whether the abundance and parallelism of the sericite are entirely due to pressure metamorphism. If the original clay contained mica flakes, a certain amount of parallelism was perhaps already developed in the shale, before any regional metamorphism took place 1.

In most cases, those specimens showing the greatest evidence of pressure are from exposures near intrusive masses, where the

l RUBEY, W. W., Lithologic Studies of Sedimentary Rocks of the Black Hills

Region: U. S. G. S. Prof. Pa per 165, p. 39, 1930.

54 DOROTHY WYCKOFF

steeper dips of the beds show rather intense local deformation. It does not appear that the sedimentary series as a whole has suffered any great amount of dynamic metamorphism.

Structure: The general directions of strike and di p are indicated on Map. 3. Mt. Gausta is composed of beds which for the most part dip about 30° southwest, but rather larger local variations in the dip occur. These seem to be connected with the intrusive rocks (which are described in a later section). At the north end of Mt. Gausta (Photograph 18), the dip changes to about 30c southeast, where a thick sheet of gabbro is intruded beneath the quartzite. Similarly, at the south end, the quartzite in Gaustaknærne dips 30° northwest, and still farther south, in the ridges immediately above another intrusive sheet of gabbro, the di p is 40-45 o northwest­northeast (Photograph 19). The ridges on the southwest side of Gausdal show steep dips - 50-60° northwest or north - just above the same sheet of gabbro; but Bonsnås and the other ridges south­west of Gausdal have, like Mt. Gausta, a general dip of 30° southwest. In the middle of Mt. Gausta, just above Langefonn 1 is a slight anticline: the quartzite south east of this point dips l 0-15 a south, or Iies almost horisontal. This may again be connected with igneous intrusives: the granite and gabbro exposed lower down on the southwest sid e of the mountain, in Gausd al.

It appears that the quartzites of Telemark have undergone complicated and widespread falding. This is indicated by the dips given for various parts of East Telemark in WERENSKIOLD's report2 (see also Map 2), but the details of the structure have not yet been worked out for the whole region. It seems plain, however, that the igneous rocks were intruded during the period (or periods) of folding, for in many cases they occupy anticlinal arches, large or small, in the sedimentary series.

Contact Metamorphism: Near the contacts with the igneous rocks, hydrothermal solutions have introduced various minerals into the sediments. On contacts with the gabbro, biotite is sometimes present, chlorite nearly always, and in greater amount. Some of the biotite is similar to that found in the gabbro, and some is weakly

I -fonn - snow-field; lange- long. This long, narrow streak of snow is a

conspicuous mark on the northeast side of Gausta.

2 WERENSKIOLD, WERNER, Om Øst-Te!emarken: N. G. U., nr. 53, Årbok for

1909, 19 10.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 55

pleochroic, with a pale brown-green color; the latter is partially altered to chlorite. The chlorite is commonly aphrosiderite, with fairly strong pleochroism, pale to bright green, and anomalous blue, purple, or purplish-brown interference colors; but some of it has weaker pleocroism and lower index of refraction; this rna y be dia­bantite or even penninite.

Epidote is abundant in some specimens; in some cases it is so abundant as to give the whole rock a greenish tinge. This is common epidote, with strong pleochroism, X very pale pinkish brown to

Phot. 18. North end of Mt. Gausta, from the southwest, showing contact between

gabbro and arkose. (At the extreme left a distant glimpse gives the leve!

of the plateau. The top of the peak is 1883 m. above sea leve!,

the tarn in the foreground 929 m.).

colorless, Y yellow, Z yellow green. The indices of refraction, deter­mined by the immersion method, are ex 1.730, � 1.750, y 1.760. The birefingence is high, 2 V large (negative). According to WIN­

SCHELL 1, this corresponds to an epidote having about 12 °/o Fe208• Colorless epidote (clinozoisite) is also present in many cases, and rarely a brown mineral (probably allanite) as small cores within the green epidote.

Calcite is common in contact zones, either as veins, or as large porphyroblastic crystals, honeycombed with inclusions of other mine­rals. The euhedral apatite and zircon, the diamondshaped crystals of titanite, and the titanite rims surrounding grains of iron oxide, are also evidently of hydrothermal origin.

t WINCHELL, A. N., Elements of Optical Mineralogy, v. 2, p. 355, fig. 291, 1927.

56 DOROTHY WYCKOFF

The "ores" in many contact specimens are undoubtedly due to hydrothermal action. Perfect euhedral crystals of magnetite (and less commonly of pyrite) sheathed in chlorite are developed. In other cases, microscopic study reveals the presence of much iron oxide in the interstices between grains, or as a secondary growth in grains which may have been originally mafic minerals of basic rocks. Common associations are chlorite-iron oxide, or zoisite-quartz-iron oxide, in a form suggesting a pseudomorph after some other mineral.

Still another form of alteration found in some contact zones is the production of colorless amphibole, probably actinolite. This may be associated with epidote, or with iron oxides alone. The needles have positive elongation, Z 1\ c about 18u; they are extremely fine, in felted aggregates replacing other minerals.

Tourmaline occurs in the sediments near the granite: it is in

small crystals, pleochroic, O blue green or greenish brown, E faint pink, yellow or colorless, O> E. Biotite, chlorite, iron oxides, and calcite are also found in the same specimens, but it is impossible to say whether they come from the granite magma, or from the gabbro, since gabbro intrusives are near at hand in every case.

Interpretation and History of the Sediments: The nature of the materials indicates that these sediments were derived from a region where volcanic rocks, very like those found beneath them, were

exposed to erosion. Such rocks cover many hundreds, if not thou­sands, of square kilometers at the present time. Moreover, studies of the structure of the quartzite formations as a whole have n ot yet made plain the location of ancient shorelines. Therefore it is im­possible to say whether the sands and clays came from the north, south, east, or west. Some general ideas as to the formation of these sediments may, however, be suggested by a reconsideration of the foregoing observations.

The lower beds contain pebbles and angular fragments, and a great amount of "sand" which is in reality comminuted fragments of dense volcanic rocks. The feldspars too were apparently derived from the same volcanic rocks: it is significant that only microcline and albite occur, and that some of the microcline shows partial replacement by albite in the manner described in the section on the volcanic rocks. The possibility that such albitization might have taken place after the formation and consolidation of the sediments seems to be nega­tived by the fact that albite does not occur either as a cementing

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 57

material along with quartz and sericite, or as a constituent of any of the veins that traverse the quartzites.

Another point of interest is the textures of the rock fragments: the original ophitic texture, or the micropoikilitic texture, is un­destroyed in many cases; in other cases, the rock fragments contain much sericite with a sub-parallel orientation like the texture of the volcanics that have been subjected to pressure. Moreover, such frag­ments, where they are found in a compressed arkose, preserve their

Phot. 19. Gaustaknærne from the southwest, showing position of intrusive sheet

of gabbro on northeast side of Gausdal.

own parallel texture, even where this is quite at variance with the direction of the sericite in the interstices of the ar kose: the parallelism of the fragments is plainly older than the parallelism of the arkose.

The obvious conclusion seems to be that at the time when these sands were being accumulated the volcanics had already been sub­jected to l) thorough hydrothermal alteration, and 2) dynamic meta­morphism. This would indicate that at long period of time had elapsed between the formation of the volcanics and the beginning of the formation of the sediments. This is in agreement with the fact that there is a large angular unconformity: the tuffs di p very steeply in a northwest or southeast direction, while the quartzite has a dip in general of about 30° southwest.

58 DOROTHY WYCKOFF

Both REUSCH and WERENSKIOLD noted this unconformity of dips 1, and WERENSKIOLD, observing in the field the breccia south of Hedders­vatn, suggested that the quartzites might be overthrust upon the volcanics. This breccia is also seen just beneath the quartzite in Gausdal. But a microscopic study of it proves that it is a volcanic flow breccia, similar to those which are seen in many places among the Javas, and that there is no reason to connect it in any way with the contact between the volcanic series and the quartzite series. In fact, the presence in it of fragments with such delicate structures as spherulites and perlitic parting proves that it could not have formed under, nor later endured, any great displacement. In the quartz­ite series also, while some specimens taken near the Iower contact show evidences of pressure, other show absolutely no such evidences.

The fact that the contacts between the volcanics and the quartz­ites are almost everywhere intruded by gabbros and greenstones makes it difficult to prove the existence of any old land surface upon which the quartzite series was laid down. But where there are no intrusives the contact appears to be transitional: i. e., the massive Iowest beds of the sediments have a composition almost identical with that of the underlying volcanics - being made up of comminuted fragments of the groundmass material, and · broken phenocrysts o f quartz and feldspar. For this reason the contact is in places almost impossible to Iocate exactly in the field, (e. g., on the northeast side of Mt. Gausta, near Heddersvatn, and in Hedders­fjell), and only by a careful study of suites of specimens taken above, on, and below the assumed contact was the relationship made clear.

This circumstance suggests that the sea encroached gradually upon a land surface which had been exposed to thorough subaerial weathering, and that the first Iayers of sediment accumulated practi­cally in situ. U pon these beds were la id down more beds of very similar material derived from the weathered surface of the land. Later on, the supply of this material diminished; or, with further transgression of the sea, this particular region was situated a little farther from the shoreline. During this time, quartz sand was de­posited, while the clay (and sericite?), derived from more complete

l REUSCH, HANS, Geologiske Iagttagelser i Telemarken, o. s. v.: Kr. Vid. Selsk.

Forh. nr. 2, 1896;

WERENSKIOLD, WERNER, Om Øst-Telemarken: N. G. U., nr. 53, Årbok for

!909, p. 41, 1910.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 59

weathering of the feldspars and fine groundmass of the volcanics of the land surface, was carried further out to sea. Still later, this fine material was being deposited upon the quartz sands. This is as far as the history of the deposition of this sedimentary series can be carried from a study of this limited area. Doubtless many oscillations took place: the presence of thin clayslate beds in the ar kose, and of arkose in the more quartzitic beds, are indications of varying conditions; and an extended study of all the Telemark quartzites would certainly reveal oscillations on a still larger scale.

After consolidation, the layers of sandstone, arkose and shale were cemented and hardened, with a moderate amount of dynamic metamorphism, into quartzites and slates. The folding revealed in their present attitude is connected with a period of mountain-making and igneous activity, when the granite and gabbro magmas were intruded throughout the region.

The Intrusives.

Description: The Granite: Gra ni te is exposed on the southwest side of Mt. Gausta. A porphyritic granite, probably genetically con­nected with the same mass, occurs just below, in the bottom of Gausdal; and the re is a smaller exposure of the same material in the little valley between Reisjåfjell and Rolinut. Both exposures of the granite porphyry contain dark xenoliths.

The granite is a medium fine-grained, light grey or pinkish white rock. Quartz and feldspar are the main minerals; a small amount of muscovite, or biotite, or both, and a Jittle black iron

oxide, can be made out with a hand lens. Under the microscope, a section from near the center of the

main mass shows a typical eugranitic texture, with quartz and feldspar as the main constituents. The quartz contains minute rounded in­clusions (n < quartz), pro ba bly liquid, and small thin needles of apati te. The feldspar is microcline, but largely altered to albite (with sericite). The albite, with typical "chess-board"twinning, is in places invaded by a myrmekitic growth of quartz. This albite-quartz replacement is even more extensive toward the borders of the mass, and in some thin sections no microcline at all remains.

The accessories - muscovite, biotite, and magnetite (with tita­nite) - have the appearance of hydrothermally introduced minerals, though it is not impossible that their presence is due to recrystal-

60 DOROTHY WYCKOFF

lization of constituents of the original magma. The magnetite is in subhedral forms ( octohedral) with ri ms or associated gran ul ar agg re­gates of titanite. The biotite is in small anhedral flakes; the muscovite in radiating fans of thin plates.

The other accessory minerals - chlorite, epidote, and calcite -are even more plainly of hydrothermal origin. They may be found in well-defined cracks, cutting a cross all other minerals; or they may penetrate and partially replace earlier minerals. Replacement of bi o tite by chlorite may be traced in several thin sections; epidote and calcite are abundant, along with sericite, in the altered feldspars. The fact that the chlorite (a bright green, strongly pleochroic aphro­siderite) and the epidote (pleochroic yellow-green, with high bire­fringence) are similar to those found in, and on the contacts of, the gabbro elsewhere, points to the gabbro magma as a source. This hypothesis is strengthened by the fact that both minerals are most abundant in a specimen taken just above the gabbro, while both are completely absent in a specimen taken higher up and farther to the northwest, where no gabbro outcrops appear.

Dynamic metamorphism has been very slight: wavy extinction of the quartz is not uncommon, but granulation is negligible or lacking. The presence of more or less parallel systems of cracks filled with secondary minerals points to a period of dynamic mave­

ment previous to, or perhaps directly connected with, the intrusion of the gabbro.

The granite porphyry has a groundmass of exactly the same character as the granite, though a little finer in texture, with con­spicuous patches of quartz-albite myrmekite. The phenocrysts, formerly of microcline, are extensively or entirely altered to albite (and sericite), with later replacements of quartz and also of calcite. Quartz phenocrysts, small and rounded, have suffered no change. The accessories - magnetite, biotite, chlorite, and epidote - are similar to those of the granite.

A section of one of the dark inclusions found in the granite porphyry shows such extensive alteration that its original character is quite lost. The fragment is made up principally of biotite and muscovite, with some quartz and a little albite; it seems to have been completely recrystallized. The biotite is most abundant at the edges, and calcite is found between this and the porphyry matrix. The presence of so much biotite indicates little except that the

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 61

xenolith was more basic in composition than the enclosing granitic magma. Whether it is a fragment from the more biotitic portions of the underlying volcanics, or from a basic intrusive rock, is uncertain: biotitization would be the result of the reaction between the granitic magma and an inclusion of either kind 1•

The Gabbro and "Greenstone": Large intrusive masses of gabbro are found as sheets under and in the sedimentary series, at the north and south ends of Mt. Gausta, on the southwest side of Bonsnås, and associated with the granite in Gausdal (see Map 3). Another large mass intruding the volcanics is exposed around the !akes Kov­stulvatn and Toskjærvatn (only part of which is shown on the south­east corner of Maps 3 and 4).

The gabbro is massive, medium to fairly coarse-grained, with a distinct ophitic tex ture. The co l or is a dark violet.grey, where the rock is composed of grey laths of feldspar and a dark ferromagnesiam mineral; or green, where the main constituents are a yellowish- or greenish-white feldspar and green amphibole and chlorite. Accessory apatite, magnetite and pyrite are visible with a hand lens. Along the borders of the main gabbro masses, and as thin sheets or dykes directly connected with them, occur "greenstones", in which the texture is very dense, and the structure generally schistose, owing to the presence of chlorite or other micaceous minerals. There are in the volcanics also numerous small dykes of the same character, as well as dykes not similar to any facies of the gabbro. The latter will be separate! y discused: their age is a matter of uncertainty.

The "freshest" specimen of the dark grey gabbro shows under the microscope an ophitic texture, with augite filling the interstices between laths of feldspar. The feldspar is labradorite, with n) balsam, and symmetrical extinction on the twinning lamellae reaching 30 " ; 2V is large and negative. The augite is very faintly pleochroic, X pinkish yellow, Y, Z pinkish grey; X/\c = 37° (or Z/\c = 53°). The birefringence is rather low, 2V about 50° and positive, dispersion weak (inclined). This seems to be an ordinary augite, perhaps con­taining a small amount of titanium. Olivine is also present, in large subhedral crystals, with the usual high index of refraction and bi­refrigence, and 2V near 90 o (positive). Apati te and magnetite are common accessories.

l BoWEN, N. L., The Evolution of the lgneous Rocks, p. 197, 1928.

62 DOROTHY WYCKOFF

Alteration has been extensive, even in this specimen. The labradorite shows the beginning of "saussuritization", with small grains of epidote along cracks. The augite is seen in all stages of alteration to amphibole: two amphiboles are produced, l) a colorless form, with fairly high birefrigence, Z/\c about 18°, probably actinolite; 2) a green-blue pleochroic form, Z/\c about 22c, Where both occur together, the actinolite forms an inner, the green hornblende an outer, zone. In some places no augite remains, only a criss-cross aggregate of actinolite, with a border of radiating green hornblende fibres. The olivine also is altered along contacts with the feldspar to a fibrous or felted intergrowth of colorless and green amphiboles; while the interior of the crystal is more commonly filled with serpentine and chlorite in minute scales. Abundant iron oxide is released by this process of alteration, and outlines the cracks of the original mineral with a black dust, or reddish-yellow stains of hematite end limonite.

There is also a large amount of black iron oxide of a different and enigmatical character: large solid grains, having subhedral curi­ously embayed forms. It is possible that these magnetite crystals formed early and were then strongly resorbed in the magma; but the way in which they are fitted into the space between other minerals, and the abundant secondary minerals associated with them, suggests a secondary origin by replacement of some earlier mineral. In this

thin section the magnetite grains are surrounded by a narrow zone of strongly pleochroic biotite, pro ba bly phlogopite: X nearly colorless or pinkish-yellow, Y, Z bright red-brown, X<Y<Z; 2V very small and negative. This is found only in intimate association with the magnetite and seems to be derived from it, or by reaction between it and the feldspar. Partial alteration to a green chlorite with fairly strong pleochroism may be seen in some places.

The further advance of these alterations may be studied in other thin sections. The feldspar is altered by saussuritization to oligoclase or even albite (at !east 95 °/o of the albite molecule), thickly filled with small grains of epidote (generally colorless, but the green pleo­chroic variety is also present in some cases). Fresh-looking albite, along with quartz and calcite, has been hydrothermally introduced: the final stage may be a sort of myrmekitic intergrowth of quartz and albite.

The augite is completely altered to amphibole, biotite and chlorite. The commonest type of amphibole is strongly pleochroic, wit X pale yellow, Y deep grass green, and Z blue green. The absorption is

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 63

X<Z<Y in the deepest colored varieties, X<Y =Z in paler varieties. It is monoclinic with the optic plane 11010, where Z/\c=22°. 2V is negative and small, not over 40-45 o (and may be still smaller -estimated at 25 - 30 o - in the darkest co l o red varieties); dispersion is strong p<u. This amphibole is usually seen in zoned crystals, the outer zone being deeper in color and more strongly pleochroic; the central part paler, with weak pleochroism, weak dispersion, and a larger 2V. The indices of refraction of the deep green material, determined by the immersion method are o:= 1.668, �= 1.684, y= 1.686. The paler parts have slightly lower indices. This is not an ordinary hornblende, for the angle Z/\c is a little too large, 2V much too small, and the pleochroism anomalous. According to WINCHELL's 1

diagram, it is amphibole intermediate in composition between cum­mingtonite, greunerite and hastingsite, nearest to the last. But the index of refraction is lower than any given by WINCHELL for hasting­sile. BILLINGS2 reports "magnesiohastingsite" with indices similar to those of this mineral, but the green color and pleochroism, as well as the size of 2V, agree belter with his "ferrohastingsite", which has higher indices of refraction.

In more high ly alte red specimens, biotite is even more abundant: either a red-bro.wn phlogopite, or a lepidomelane having strong pleo­chroism, X pale yellow, Y, Z dark olive brown, X<Y = Z. It is apparently uniaxial and negative. It is intimately associated not only with the magnetite, but also with the green amphibole, and seems in many cases to be formed by alteration of the latter mineral. Minute hair-like needles, probably rutile, are found as inclusions, arranged

in lines intersecting at 60 °. Pleocroic haloes are large and dark, both in the amphibole and in the biotite: the mineral at the center, where it is visible, has a high index and high birefringence; it is presumably zircon.

A further alteration that may be traced is the change of biotite into chlorite. This mineral has a strong pleochroism, X colorless or pale yellow, Y, Z blue green; it is apparently uniaxial and negative. A somewhat less cornmon positive form has X, Y blue green, Z color­less or pale yellow. The � index of refraction, determined by the immersion method, is 1.630; o: and y could not be directly determined,

I WINCHELL, A. N., Elements of Optical Mineralogy, v. 2, p. 209, 1927. 2 BILLINGS, M. P., The Chemistry, Optics, and Genesis of the Hastingsite Group

of Amphibo\es: Am. Min. v. 13, p. 287, 1928.

64 DOROTHY WYCKOFF

but the birefringence is low in every case, and the dispersion strong, producing anomalous blue or purplish-brown interference colors. Pleochroic haloes, apparently inherited from the biotite or horn­blende, are sometimes seen. This is aphrosiderite, similar to that occuring as a hydrothermally introduced mineral in the other rocks of the region. Ultimately all the hornblende and biotite may be replaced in this way.

Large patches of magnetite, with curious angular forms suggesting a pseudomorphous growth after some other mineral, are common. In many cases the magnetite (which is titaniferous, or is possibly ilmenite) is surrounded by titanite rims, and complete replacement by titanite may be traced. Pyrite, in euhedral crystals, is found in some specimens. It is commonly surrounded by aphrosiderite, but shows no signs of decomposition.

The rock finally resulting from this series of alterations is made up chiefly of chlorite, with patches of granular quartz, albite and epidote. The accessories are apatite (which remains unchanged, though possibly more has been hydrothermally introduced), granular titanite (commonly retaining a core of titaniferous magnetite or ilmenite), and freshly introduced pyrite. Calcite is abundant, in cracks, or as porphyroblastic crystals including or replacing earlier minerals, especially feldspar.

In the gabbro itself evidences of pressure are extremely slight or entirely lacking, but the highly altered chlorite rock grades into smaller sheets of "greenstones" where a parallel texture is quite distinct. This parallelism may be due to the original texture of these smaller bodies, or it may have been subsequently developed. In these border "greenstones" (num bered "l " on Map 4) the most im­portant mineral is chlorite: it is in some cases aphrosiderite, in others a paler colored variety, with much weaker pleochroism, � index of refraction 1.60 ± (determined by immersion): this may be dia­bantite or even penninite; both positive and negative forms occur. Pale colored biotite, and minutely fine shreds of a mineral which may be sericite or serpentine 1 are also abundant. The interstitial material is mostly quartz; in some cases a little albite can be identified. Veins, patches and porhyroblasts of calcite are conspicuous. The

l Serpentine (antigorite) may be considered as an end-member of the chlorite

series. WINCHELL, A. N ., Elements of Optical Mineralogy, v. 2, p p. 373 ff., 1927.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 65

accessories are magnetite and titanite, occurring either separately or associated as in the gabbros.

Where alteration is less intense, or of a slightly different character, the ophitic texture is not completely destroyed, and the rock is composed of laths of albite, with much actinolite and a little chlorite (diabantite ?), and a large amount of iron oxides and titanite. The iron oxide seems here to be a released mineral, while in other facies of the rock a part, at !east, of the iron enters into the composition of the strongly colored green amphiboles and chlorite.

The Smaller Dykes: Within the area of the volcanic rocks occur many "greenstone" dykes (numbered "!" of Map 4), of a character very similar to the border facies of the gabbro sheets The minerals are essentially the same, though the texture is commonly very fine and somewhat schistose. These dykes apparently occupy flssures following the trend of the main joint systems in the volcanics. Field exposures show that these flssures are nearly vertical in some cases. The "greenstones" are especially numerous along contacts between the different types of Javas, or between the Javas and the tuffs.

At two or three point along a line running northeast from Reisjå­vatn are found exposures of a greenstone porphyry, which has in the fleld an appearance quite different from any part of the gabbro. The groundmass in den se, greenish grey; small flakes of biotite are visible with a hand lens. The phenocrysts are of feldspar, up to 2 cm. Iong, with indistinct rhomb- and Iath-shaped outlines. Where weathered the rock is much greener, probably due to the presence of chlorite, and the white feldspars stand out in relief.

Under the microscope, the rock shows a mineral composition very similar to parts of the gabbro. The phenocrysts, probably originally of labradorite, are composed of oligoclase, with patches of intergrown quartz and albite, and small grains of epidote and biotite. The groundmass is composed of small anhedral grains of albite, quartz, epidote, green amphibole, biotite and chlorite, with accessory magnetite and a little titanite (leucoxene). The rock shows no schist­osity, but is traversed by many cracks which are fllled with green epidote, quartz and iron oxides.

One isolated Iedge (numbered "2" on Map 4), rising from the glacial till of Råen, l km north of Kovstulvatn, exposes a dense greenish grey rock, thickly sprinkled with small shining needles of blackish green hornblende.

Norsk geo!. tidsskr. XIII. 5

66 DOROTHY WYCKOFF

This shows under the microscope a groundmass similar to that of the preceding rock, but finer in texture. The phenocrysts, or porphyroblasts, are of green amphibole like that of the gabbros, but unzoned, strongly pleochroic throughout. The crystals are nearly euhedral, with prismatic forms and steep terminal faces, but are filled with inclusions of the groundmass materials. Apatite and black iron oxide are abundant accessories. Secondary quartz is present in large amount, in veins and small sharply defined patches, possibly representing replaced earlier minerals. This is apparently very like "amphibolites" described by WERENSKIOLD 1 .

The character of both these rocks suggests a close association with the gabbro, though no actual connection can be traced in the field. Probably they are of the same age as the chlorite schists; and all, while not parts of the same intrusive body, may be assigned to a single period of igneous activity - that during which the main masses of gabbro and granite were intruded.

There are also among the volcanics small dykes (numered "3" on Map 4) of black biotite schists, which are so highly altered that little or nothing of their original character can be discovered. A thin section of this material shows a groundmass made up of olive-brown strongly pleochroic biotite, with some quartz, epidote and albite. There are zones, streaks and patches where the biotite is subordinate, and

the texture of the albite and quartz is very similar to that in some of the volcanic rocks. Small phenocrysts of albite, and larger ones of gran ul ar quartz (replacing earlier phenocrysts ?) are numerous. These dykes are small and are connected with zones of intense biotitization. Their attitude, where exposures made it possible to determine this, is steeply dipping or vertical. It seems possible that these represent a much earlier intrusive series which cuts the volca­nics alone, and may be connected with an earlier period of meta­morphism than that in which the quartzites were also involved.

Jnterpretation and History of the lntrusives: The age relations of the granite and gabbro are uncertain. As stated above, the presence of hydrothermally introduced minerals like epidote and chlorite in the granite near the contact with the gabbro, and the absence of such minerals farther away from the gabbro, would seem to indicate that the gabbro is younger. On the other hand, in a few places tourmaline

I WERENSKIOLD, WERNER, Om Øst-Telemarken: N. G. U. nr. 53, Årbok for 1909,

p. 29, 1909.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 67

crystals are found in the "greenstone" border facies of the gabbro, which would seem to indicate that at !east some of the sheets of gabbro were intruded before the granites. Unfortunately no distinct cross-cutting contact has been discovered. On the southwest side of Mt. Gausta the slopes are so covered with scree from the high peak above that all contacts are covered except the comparatively uninter­esting contact of the upper greenstone with the overlying quartzite. In Gausdal, the contact of the granite prophyry with the gabbro seems to be transitional, though the dark inclusions in the former may be from the gabbro. WERENSKIOLD 1 thought that the granite and the basic rocks found together in other parts of Telemark must be essentially comtemporaneous, produced by differentiation from the same magma. Some such history of differentiation and hybridism may have been the case als o in the Mt. Gausta region: the separation of two magmas which might later be intruded more or less simultaneouly would be most likely to take place during tectonic movements of the region; and the relations of the intrusives to the folds in the quartzites indicates that folding and igneous intrusion occurred at the same time.

Both granite and gabbro show extensive alteration, with the production of typical "synantetic"2 minerals. In the gabbros biotite rims occur around iron ores in contact with feldspar; and augite and olivine are surrounded by "coronas" of colorless and green amphi­boles. In the granite occurs "true myrmekite, which is an intergrowth of plagioclase and vermicular quartz, and has originated as a crystallo­blastic product replacing potash feldspar" 3. All these mineral asso­ciations have been discussed by SEDERHOLM: the production of synantetic minerals may be regarded either as a final stage in the crystallization of a rock body, when residual solutions from the original magma are still active and react with minerals al ready form ed; or as a later event, when the rock body is subjected to the action of hydrothermal solutions from another source. SEDERHOLM believes that the myrmekites found in granites are perhaps more usually (though by no means always) deuteric in origin; and it is of interest to add here his observation 4 that in man y known cases "these reac-

l WERENSKIOLD, WERNER, op. cit., p. 28.

2 SEDERHOLM, J j., On Synantetic Minerals: Bull. Comm. Geo!. Finlande,

nr. 48, 1916. 3 SEDERHOLM, Op. cit . p. 13 1. 4 SEDERHOLM, op. cit. p. 128.

68 DOROTHY WYCKOFF

tions have been favored by the existence in the vicinity of semi-molten basic rocks", either as inclusions or as intrusives. In the Mt. Gausta region, too, the granite porphyry in Gausdal, which has a transitional contact against the gabbro, and contains numerous dark inclusions, shows an unusually great amount of typical myrmekite. He is more inclined to regard the formation of "coronas" about the minerals in basic rocks as "secondary" in a majority of cases, though he reports instances where such changes have been brought about "by gases or liquids belonging to the magma of the rock itself" 1•

The change of biotite to chlorite may be considered as a late deuteric change, or as the first of the series of hydrothermal changes which are usually called "secondary"2. As a matter of fact, the later stages of hydrothermal activity, with successive introduction of chlorite, quartz, calcite and "ores" can be traced in thin sections, and can be seen on a larger scale in veins cutting the (chloritized) gabbro. In Gausdal, by the small tarn at the extreme northwest end of the valley, are found several veins of white quartz, up to l or 1.5 m. in width, which carry siderite, pyrite, chalcocite, along with secondary hematite and a little malachite and azurite3.

It is then not impossible that all the changes observed in these rocks may have been produced successively during the progress of a single igneous cycle. But this argument does not exclude the possibility that hydrothermal alteration may have occurred at some later time.

Finally, an interesting point is raised by the chemical composition of some of the minerals involved in these changes. The replacement of potash feldspar by quartz-albite myrmekite is only a "special case of albitization"4. The considerable amount of freshlooking albite replacing some of the calcic plagioclase also indicates that the hydro­thermal solutions must have been rather rich in soda. Moreover the green amphibole which replaces the augite and olivine is apparently nearer in composition to hastingsite than to ordinary horn blende:

l SEDERHOLM, op. cit., p. 39.

2 BuTLER, B. S., lnfluences of the Replaced Rock on the Replacement Minerals

Associated with Ore Deposits: Econ. Geol. v. 27, p. 4, 1932. 3 These veins have been "mined"' by local people , and l was told in Rjukan

that a small amount of gold had been extracted. It hardly seems a promising

"prospect"', however.

SEDERHOLM, op.cit., p. 140.

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 69

and there is no reason to suppose that the original augite contained soda. Hastingsite has been found as a pyrogenetic mineral in alkalic rocks of many regions. W ATSON 1 has found it as a reaction mineral forming an outer zone about augite in alkalic rocks from the San Carlos Mountains in Mexico. In this case the hastingsite formed as a result of reaction between early formed crystals and the residual liquid in the same magma. In the Mt. Gausta region, hastingsite seems to have formed as a deuteric (or possibly as a hydrothermal) mineral, by reaction between augite and later soda-bearing solutions, which may (or may not) be derived from the same magma.

The significance of this fact cannot be rightly estimated without much more extensive petrologic work in the region, and numerous chemical analyses. It seems worth while, however, to point out the possibility that even the pre-Cambrian magmas of southern Norway may be found on further study to possess a slightly unusual chemical composition, high in soda, indicating an early differentiation of an alkalic petrographic province in this region.

Summary and Conclusions.

The rocks of the Mt. Gausta region are of pre-Cambrian age, and may be di vid ed in to three main groups:

l . "Porphyroids". 2. Sediments - quartzite, arkose, and slate. 3. Igneous intrusives- gabbro, "greenstone", and granite.

The "porphyroids" are the oldest. The present paper presents detailed evidence to show that they are all of volcanic origin - tuffs and Javas - and endeavors to trace the sequence of the metasomatic changes which have taken place. These rocks, in spite of their great age and the intense alteration which they have undergone, preserve to a remarkable degree their original textures and structures. The coarser tuffs show angular fragments and shards of glass, now re­placed by al bite and quartz; the finer tuffs have an evenly graded "varve"-like structure, pointing to deposition, probably by water, under rapidly alternating conditions. The Javas show traces of flow

l WATSON, E. H., Personal communication, 1932.

70 DOROTHY WYCKOFF

structures, and well preserved resorbed phenocrysts, skeleton crystals, and lithophysae. In the flow breccias of the region occur fragments with such delicate structures as spherulites and perlitic cracks still intact; and one large xenolith in a flow breccia has perfectly preserved ripple-marks.

In all these rocks, however, there has been a wholesale replace­ment of the original substance: potash feldspar has been replaced by albite, and albite again by quartz. The micropoikilitic ground­mass, characteristic of ancient Javas in many parts of the world, and sometimes thought to be due entirely to devitrification, seems here to be the result of metasomatic replacement. There are traces of at !east two, and possibly more, subsequent periods of hydrothermal activity. The first introduced chiefly biotite, iron oxides and quartz: this may be tentatively correlated with the highly altered basic intrusives found among the volcanics. Another period, which can be connected with the intrusion of the gabbros and greenstone schists, introduced chiefly epidote, chlorite, quartz, calcite and "ores". Tour­maline, and possibly some other minerals, may be traced to solutions accompanying the less extensive granite intrusives. Dynamic meta­morphism has been active, but not intense enough to obliterate the textures and structures of the volcanic rocks.

There appears to be an unconformity of considerable importance between the volcanics and the sediments overlying them. The chief arguments for such an interpretation are as follows:

l. There is a large angular unconformity: the tuffs di p steeply northwest or southeast, or are locally vertical; the sediments have a general dip of about 30° southwest, though there are local variations due to igneous intrusions.

2. The character of the contact suggests the accumulation of the lowest be ds of the sediments practicall y in situ, during the gra dual submergence of a deeply weathered land surface.

3. Fragments of the volcanic rocks in the sediments show that before being eroded the volcanic rocks had undergone both meta­somatic replacement and dynamic metamorphism.

Granite and gabbro (with greenstone border facies) occur as intrusive sheets in the sediments, and along the contacts between the sediments and the underlying volcanics. A few small irregular masses of granite, and larger masses of gabbro, are found in the volcanics. The many small dykes of chlorite schist found in the

GEOLOGY OF THE MT. GAUSTA REGION IN TELEMARK 7 1

volcanics are probably connected with the gabbro, but same of the biotite schists may be older. Both granite and gabbro show extensive alteration. The granite has its original potash feldspar almost com­pletely replaced by albite and quartz, with typical myrmekite, and contains hydrothermally introduced biotite, epiqote and chlorite. The gabbro, apparently originally an augite-labradorite rock, has the labra­dorite largely replaced by oligoclase, or even al bite, with epidote; and the augite by a green amphibole, near to hastingsite in compo­sition, which may be in turn replaced by a green chlorite (aphro­siderite). Whether these synantetic minerals are to be attributed to deuteric changes, or whether they are more probably due to later hydrothermal alteration on a regional scale, is not certain. In either case, there is in these pre-Cambrian rocks some evidence of that alkaline character of the magmas or magmatic solutions of South Norway which is so strikingly apparent in the later (post-Silurian) igneous rocks of the adjacent region of the Oslofjord.

Acknowledgements. The field work upon which this paper is based was done during

the tenure of the Fanny Bullock Workman Fellowship from Bryn Mawr College (in l 928-29) and of a Fellowship from the American­Scandinavian Foundation (in 1929-30).

In the spring of l 929, Professor V. M. GOLDSCHMIDT, of the Mineralogical Institute in Oslo, suggested this area as a problem for a dissertation, and discussed with me same of the preliminary field work. Dr. OLAF ANTON BROCH, Dosent at the University in Oslo, gave me many helpful suggestions during my study of part of the material in the winter of 1929-30. Dr. EDWARD H. W ATSON super­vised the final preparation of the paper at Bryn Mawr College in l 931-32; I am indebted to him for much valuable discussion of the problems involved, particularly those relating to the metasomatism of the igneous rocks. I am also indebted to Dr. ELEANORA B. KNOPF,

of New Haven, Connecticut, for examining a suite of thin sections of the volcanic rocks and giving me her opinion on same points connected with them; and to Dr. FLORENCE BASCOM, of the United States Geological Survey, for reading and carefully criticising the entire paper.

72 DOROTHY WYCKOFF: MT. GAUSTA REGION IN TELEMARK

The Norwegian Geographic Survey kindly allowed me to use for my field work unpublished topographic maps of the Mt. Gausta region. Of the thin sections used for the petrographic study, forty­five were made at the Geological Museum in Oslo and lent to me, by the kindness of the Director, Professor jAKOB SCHETELIG, for the completion of the work in America; and about forty more were provided by the Department of Geology of Bryn Mawr College. The photomicrographs were made by Miss LIL Y MoNSEN at the Geolo­gical Museum in Oslo.

Contents. P age

lntroduction............................................................ l Location and Geography of Telemark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . l Early Descriptions of the Rocks of Telemark... . . . . . . . . . . . . . . . . . . . . . . . . 2 The Scandinavian Pre-Cambrian. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 The ''Telemark Formation'" . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

The Mt. Gausta Region.................................................. 12 Physiography ............................................ . . . . . . . . . . . . 12

Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IS Distribution and Classification of Rock Types . . . . . . . . . . . . . . . . . . . . . . . 15 The Volcanics.................................................... 18

The Tuffs........................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Interpretation and History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

The Lavas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Subdivision into Types................................. . . . . 29 Description of Types........................................ 30

Type l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Type 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Type 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Type 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Type S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 The Flow Breccias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

lnterpretation and History of the Volcanics .................... 47 The Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Description - Arkose, Quartzite and Slate . . . . . . . . . . . . . . . . . . . . . . . 50 Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Contact Metamorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Interpretation and History of the Sediments . . . . . . . . . . . . . . . . . . . . . . 56

The I ntrusi ves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

The Gran i te . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 The Gabbro and "Greenstone" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1 The Smaller Dykes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Interpretation and History of the Intrusives . . . . . . . . . . . . . . . . . . . . . . 66 Summary and Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Printed january 23rd, 1933.