195

EXOURSION TO BANGOR, SNOWDON, HOLY HE AD, &C.

ON J UL Y 23RD, 1 8 8 3, AN D FIVE FOL L OWING DAYS.

D irectors :-The PRESIDENT, and P rof. T . Mel\:. H UGHES, M.A.,F ,S.A., F .G. S. , President Chester Soc. of Natural Science .

(Repol't by Prof. IhJGHEs.)

The excursion which the Associat ion did me the honour to inviteme to accompa ny, as co-director with the President , had for it sprinc ipal objec t th e investigation of the mass of Arcbrean rocksin the neighbourh ood of Bangor and Caernarvon and its relat ions tothe overlying Cam brian.

Th e secti ons were g enera lly worked in the descend ing order ofsuccession of the st rata, and the same plan will be adopt ed in thisreport, th e aim of which will be t o place on record th e factsobserved so th at members who j oined the party can most easilyrefer hereafter to the sections seen by themselves and the localitiesvisit ed, and follow most readily any interpretat ions placed uponthem in future discussions.

It was of course of t he first imp ortance to settle what were t hebasement beds of the Cambrian, and how th ey va ried in closelyadjoining areas ; for it was t houg ht th at, if , by following the bellsdown from kn own and oft en fossiliferous rocks, we could esta blishany tol erably constant horizons, then, whatever difficulti es mightarise from th e int ercalation of other rocks here and there, wesho uld at any rate geuera lly kn ow when we had got to th e bottomof the ser ies.

Th e course of our wanderings t ook us up into much higher bedsof the Cambrian and even over a considerable thi ckness of Carboni­ferou s, and these often furn ished by their local variations importantillustrations of many points of difficul ty in the older rocks.

W e counted down all th e beds from th e t op of Snowdon, wherewe were on Bala Shale with Orthis flab ellulum, of which we foundpl enty of specimens in place close to the little inn on th e Sumwit.I n descending we crossed alt ern ation s of volcanic agglomerat es,ashes and shales as far down as th e H alf-way-house. A strongvolcanic breccia forms a conspicuous feat ure j ust in front of thehouse, and was especially interesting to our party as it closely

resembled some of th e Archrenn volcanic rocks we had passed over

196 EX CURS ION TO NORTH W ALES.

ncar Bangor. Below thi s thovolcanic material soon becamequite subordinate, and we foundonly black shales passingdown into banded, flaggy bedswhich rested on a thick bed oftough quartz grit. In th eblack shales we found grapto­lites, perhaps Climacoqraptuscalatus, but the specimenswere not sufficiently well pre­served for us to feel sur e onthis point. We had nowreached the disturbed groundncar the foot of the mountain ,and the only point we made outclearly was th at th e grit rolledover and over along th e highbosses which form the summitof the r idge flanking L lynPeris at its N.W. end. Thedetails of th e lower groundhave not yet been th oroughlyworked out by any one ; butwe had above us an unbrokensuccession, from fossiliferousBala, across other fosiliferousbeds, suspected to be Arenig,down to the well-marked gritin front of us, as shown inF ig. 1.

In thi s series th ere was noplace for th e red, green, andgrey slaty rocks of the Llan­bcris Quarries ; for we werestill far above them; but it

was known that a very considerabl e thickness of such beds hadto be traversed, and below th em other slates and slaty mudstonesbefore we reached the great conglomerates which here, as every­where, occur at th e base.

E~

"...."'0Cls.gse::!

tI')e-, ,;r:

.2 ,. '"~ s

't~ ::~

;;:

".~ClI..:t:o

Ii:;

EXCURSION TO NORTH WALES. 19i

Next to take the section south of the hills south of Bangor.Near Minffordd there are black shales with traces of graptolites,and on the hill up the road to the north of them are banded sandyflags, behind which, at some distance to the north, the basementconglomerate crops out. Following the black shales to the east wefind them well exposed by the road from Bangor to Llandegai j

while, close to the Penrhyn Arms, the banded sandy flags are seenin the road immediately before the hotel, and up the hill by thegardens in front of it, dipping under the black shale. In the de­scending section, along the cliff under the Penrhyn Armsgarden, a succession of grey felspathic mudstones come out fromunder the banded flags and black shales. Bearing more to thewest and following the series across the Recreation Ground wefound that the black shales (a10) passed down into banded flags (a9)

which in their turn passed down into red and grey sandy shalesand felspathic mudstones, as shown in Fig. 2 ; while in and abovethe road, near the west entrance to the Recreation Ground, andapparently coming out from below them all, we saw the coarse base­ment conglomerate of the Cambrian (a1) , here made up almostentirely of pebbles of quartzite and felsite.

Fig. 2.-Diagram Section E. and W. across Bryniau Bangor,Length of section about I mile. ('1'. McK. H.)

BryniauBangor.

b

Recreation Ground.

b Bangor Beds (Upper Pebidian).41 Conglomerate and Sandstone. Basement

Bed of Cambrian.42 Sandy mudstone.43 Finer felspathic mudstone.44 Pale green felspathic mudstone.4S Cleaved and contorted felspathic mudstone.

a6 Purple and green fine mudstone and slate.aj Fine sandy flags like the Bray Beds, purple

and here and there green, passing up intobrown.

a8 Evenbedded thin g-rey sandstones.09 Banded flags passing up into.aloBlack Shales.

J98 EXCURSJOX TO NORTH WALES.

Between the RecreationGround and the cliff, nearGarth Ferry, are several irn-

~ vi portant faults, the result of"B which is that the black shales

~'" again-0 are met with on the"o ~ E east of the pier, passing down

.: -0 -0~ " into banded flags. Thoughe -0

' 0 0 "E§ o ~ there interruptions~ c are somet:::l :: -:,),, - "<, ... ~ :3 1: by faults which III the cliffc o iii r3 bi)

"'"~ "'0 >.fii .- section throw out

>:: :r: C tJj v. aJ may some

~ ~~ CIit' g.::: of the series, still, as the~ ~~ ti ~_ u

~c a::~ .... _

faults are oblique to the strike,., - ~].: '§ E.~ .....:.

'".... t. "'C .... u..o ..:..:: the beds can be traced'Vi ~ ~E ::a 2 up

'<; ~ui =: ;l:l U ;l:l and down beyond their In-

:; '2 :~"6~ ~ fiuence, and immediately~ - west

'c> g '0 s-, of the pier the banded flags are

~ ~:;; ",- seen to pass down into lenti-

"E c-:l "-e c .g :E

cular beds of exceedingly tough.,0

;l:l ~~ ~to'~ ::::" 0::::"§ U .~

~ coarse grit and conglomerate" e ~ Z t"<; rfl <:l

~'0

E :> = Q.., composed almost entirely of" ~ 0 '"

'" .- ~

-5 c-:l ~ - '~ quartz. This again:; ;l:l Q..e~ passes.s ~

.....,~~ :: down into a series consisting of<:: OJ ~ p..o ~

...leX f,:) .c ~-~ felspathic mudstones very likeI ~ c-:l

'e ' :"' 0 ::::s those the Recreation'" ~

-0 ~ ... C'" onc N

til "c-:l r; ~c ·c Ground, which pass into the

t.: "" g ,..:..§ B ~ ~~ t .~ ~ eb ~

coarse conglomerate which,0: though perhaps repeated some-o 81: ::: r. E8B E:: ·;.g "'0 what by faults, is stilI en-eo ~ ~ C'" ~ ::

c u _ ~:: ~ dently of great thickness, and8 ::': 8;; N '" forms the cliff up to the great

" '" fault that brings on the Car-boniferous, as seen in Fig. 3.

In the banded beds, close to the pier, we found a Lingulella,probably L. lepis.

All basement beds are, from the nature of the case, apt to be veryvariable deposits. So as we traced the base of the Cambrian fromBangor, westward to Caernarvon, we found a considerable difference

EXCURSION TO NORTH WALES. 199

in its character, but fortunately no important change occurred with­out our having had some previous indication of its coming on.

Following the Conglomerate along the south flank of BryniauBangor, we found here and there in the rough ground east ofNantgwtherin that the coarse conglomerate, made up chiefly ofpebbles of quartzite and felsite, was occasionally in part replaced bya grit consisting mainly of quartz. Having crossed the disturbedground near Tairffynon, we picked up the conglomerate again onthe road up the hill to Brithdir, and here again we found a coarsequartz grit associated with it. The conglomerate was seen here andthere for some distance to the west, until near Llanddeiniolen it wasfound strongly developed and quarried close to Tymawr. Behindthe conglomerate and dipping under it was a bed of coarse gritcomposed chiefly of quartz, but containing also a large proportionof small fragments of felspathic rock. This section was perhaps themost important of all we visited, for in it we saw a passage from thequartz grit into felspathic grit, and the felsite-conglomerate over­lying the grit. In the strike of this grit and conglomerate we camefirst of all to Archsean rocks, and then to black and red shaleswhich underlie black 01' banded shales similar to those marked a~

and a10 on Fig. 2 and a4 and a5 on Fig. 3. The features of theground also suddenly changed, and it was clear that we here crossedsome very considerable faults. We had not time or data for workingout the exact pattern of these faults; and, moreover, we were seekingfor evidence as to the sequence of the rocks where the succession wasclear, and not working out the structurc of the district. The grit wasthrown back to Careggoch. It was quite a conglomerate in places,and had hardly any trace of felspathic rocks in it. In the fields in frontof it, on the south, black shales appeared here and there. Followingthe strike towards Caernarvon, we found the quartz-conglomerate ina small quarry in a field above Yscuborwen, about a quarter of a milenorth-cast of Twt Hill. There were hardly any traces of felspathicrocks like those in the grit of Tymawr, but it contained more frag­ments of mica-schist, jasper, etc. The granitoid rock was not seenin the quarry section, but cropped out immediately behind it at thenorth-east end of the boss. The Conglomerate passed up into even­bedded sandstones which were clearly exposed along the hedge infront of the quarry.

Proceeding along the new road, which runs for a short distance

200 EX CURSI ON TO NORTH WALES.

....~ ~ - ........ - _ ...... ......., ,-v ' . ... . . ... \; , ..... ~

.._ ~-- -_.-

r "

r I I-r .... - \

r \.t -,;. ~ I ' ....- - ,- ~ ,

I r I J

along the top of th e hill above th e main Bagnor and Caernarvon road,we came to a quarry in th e granitoid rock, t he south boundary ofwhich almost exactly coincides with a fault, so that th e face of rock

in front of us, 8S we look eddirectly into the quarry fromthe road, consist ed, at thesouth-west end of th e qnartz­

/, conglomerate, while th e quarryended abruptly on our r ightat a fault partly filled by adiabase dyke.

Crossing a hedge to th eS. W. we came to anoth erquarry in a field in which the

d, brown sandstones of the Cam-brian Basement Beds wereseen passing down into the

1~~~~~=7~~71 (' quartz - jasper - conglomerates,~ which were seen res t ing

directly upon th e granitoidrock. This Conglom erat ehad been traced still furtheralong the strik e to near th echurch at the other end of

It. T wt H ill.No high er beds of the Cam­

brian were seen near thesesections, but just outside th etown, south of the Castle,Arenig Beds are well ex-posed along the road andriver. Th ese have becn de­scribed by Mr. Marr," whohas given a list of fossils from

th em.So, reviewing all the sec­

tions seen, we made out thefollowing sequence, as shown

in the section, fig. 4.

* I Quart. J ourn. Geol. Soe.,' Vol. xxxii , p. ] 34.

EXCURSION TO NORTH WA.LES. 201

Notes of Sections seen.

a. Shales on the top of Snowdon,with Orthis fiabellulum;

b. Volcanic Series on Snowdondown to below the Half.wayHouse.

c. Shales below the Half-way House.d. Blaok Shales, with Graptolites

(Olimacogmptu8 ccelatus 1). Onthe flanks of Snowdon, belowthe Half-way House; at Llan­rug, three miles east of Caer­narvon; Pont Seiont, south ofOaernarvon ; and along thostream which runs from belowFfriddodd to the LlandegaiRoad near the Penrhyn Arms,Bangor.

e. Banded f1aggy beds close to thePenrhyn Arms, Bangor; on thehill east of the RecreationGround and north of the softblack shales above mentionednear Ffriddodd; close to GarthFerry Pier, where they yieldedLing uletla lepis (i')

f. A. coarse, tough g'rit seen west ofthe Pier at Garth Ferry; Onthe flanks of Snowdon, abovethe road between the Half-wayHouse and the Victoria Hotel.

a. Black shales above Victoria Hotel,Llanberis, doubtfully referred tothis horizon.

11,. Bedded felspathic mudstones.Very variable beds, much thickerin the Llanberis district, wherethey include all the slate-bearingseries of which only a few thinrepresentatives are found in thepnrple beds of the RecreationGround and of the cliff west ofGarth Ferry Pier. The seriesis seen also in the cliff below thegarden of the Penrhyn Arms,Bangor. It has almost entirelydisappeared in Angle8ey.

Probable E'luiv'1lents elsewhere.

Middle Bala.

Ditto.

Lower Bala.Upper Arenig.

Lower Arenig'.

Garth Grit, probably of the same ageas the great grit which runs bythe farm of the same name(Garth) near Portmadoc.

PTremadoc.

The Menai Series comes where weshould expect to find the Lin­gula Flags, Menevian and Har­lech, but no exact correlationis possible with only the databefore us.

202 EXCURSION TO NORTH WALES.

Notes of Sections seen,

k. The Conglomeratic Series, whichare, of course, from the natureof the case, also very variable.In the Bangor district consist­ing chiefly of fragments of fels,pathic rocks and quartzites, butas we follow them to the south­west containing more quartz,either in alternate beds ormixed up with felspathic rna.terial,

Probable Equivalents elsewhere.

Basement Beds of the Cambrian, notnecessarily of the same ageeverywhere.

Thus it appeared to us tllat although the basement Oonglomer­ate and grit were very variable, yet, by eounting down from bedsabout which there was no doubt, through a series which had afairly uniform sequence, we could often make out with certainty whenwe had reached the basement beds, and we commonly found thefelspathic pebbles and grains mixed up or interbedded with the quartzpebbles and grains in such a manner that it was clear we must referthe variation to difference of origin not involving difference of age.We, therefore, next enquired on what did these basement hedsrest?

In the Bangor district at Ffriddodd, and near the Rccl'eationGround, and along both flanks of the hills fr()m Bryninu Bangiwto the south-west towards l\1inffonld on tlJ{' 011(' side and CnJ"('g' Hwf,ton the other, we found under the Conglomerate rhi('fly nslu-s ana

various coarser and finer febpathic rooks, mainly composed of vol­canic ejectamenta. As we followed the basement !Jeds of the Cam­brian to the south-west we found cropping out from below them thegreat felsitic series of Brithdir and Dinorwig. Here the CnmbrianConglomerate, being transgressive over the edges of the Archa-en

rocks, had overpassed the Bangor Volcanic Beds, and was restingupon another and different series, Agreeing with this we found atTymawr, neat" Llanddeiniolen, a much larger proportion of thepebbles in the Cambrian Conglomerate derived from these felsiticrocks.

Then, after crossing the Llandd-iniolen faults, we had to pick upthe line of the base of the Cambrian much further north, and thenext series seen coming out from under the basement beds was the

EX CURSJOX TO NORTH WALES. 203

fine sub-crystalline rock of Crug. We followed th e upper roadtowards Caernarvon, and in th e quarry in a field near Yscuborwenwe saw that the Conglomerate rested upon a coarse granitoidrock, which formed th e great er part of the rest of the ridge, alongthe north flank of which the Conglomerate was seen at intervalsto the end of Twt Hill.

-0

IIII

,c

So we ascertained that a regular sequence could be made out14

204 EXCURSION TO NORTH WALES.

among the rocks which we come upon below the Cambrian, andthat it was as follows, in ascending order (see Fig. 4) :-

A I. The coarse granitoid rock which runs near Caer- t 81 } . .narvon, by Llysmeirion to near Dinorwig. f ~ I Dimetian,. a

.,11.2. The finer rock, in which the crystalline strUO-1 ~ 1tnre is not always apparent, but which often a r r;shows an approach to the characters of AI, and a I§ Isometimes looks like BI metamorphosed. Near o.'l •Crug, north-east of Caernarvon. J J r Arvonian.

BI. Rich black, purple, and pink quartz felsites'l t11 Ioften a quartz-Ielspar-porphyry, sometimes rS'brecciated. A great group, consisting chiefly I ~ Iof lavas. Near Dinorwig, N,E. of Cmg, J oil' ~ ~

• r~ JBa. Voloanic ejectamenta, from fine ash to coarse } ~ I~ }

agglomerate, but with few lavas. Near 'g. Pebidian.Bangor. ~ J

The microscopic character and composition of these rocks havingbeen described by Prof. Bonney and others, we confined our atten­tion to the questions of the succession of the rocks, and satisfiedourselves (1) that there was an obvious sequence in the rocksagreeing with these details of internal structure, and that anascending section could be traced from Caernarvon to Bungor ; and(2) that, whatever difficulties might arise from the probability ofthere being altered intrusive rocks among the lowest members ofthe group, and whatever difference of opinion there might be as tothe relations of these older rocks to one another, there certainlycould be no doubt that the Cambrian was transgressive over themall, and that the variations in the character of its basement bedswere just such as might be expected from the character of that partof the underlying group which lay behind it in such a position aswould represent the shore from which shingle drifting with prevalentsouth-west winds would be derived.

Some collateral evidence was gained ill our trips to Anglesea.The clear Carboniferous sections of Penmon and the glacial depositsof Beaumaris having been examined, some of the party returnedto the more difficult ground between the cemetery and Garth Ferry.The section along the new drive to Baron Hill was followed fromabove the cemetery, and felspathic mudstones and grits were foundin regular succession about half-way along the drive, where they

EXCURRlON TO NORTH WALES. 205

passed down into felsite-conglomerate. This was cut off by afault known to be running up from the shore in this direction, andgreen Archsean schists are seen on the west of it. These appear tobe in some places cut off by faults, and in others to be over­laid by Cambrian grits and fossiliferous shales along the shoreof the Straits to nearthe end of Garth Ferry, as shown in thetwo following sections :-(Figs. 6 & 7.)

Fig. 6.-Section on shoreabout zSoyards N.B. ofGlyn Garth, Anglesea, oppositeBangor.

{

b.Cambrian c.

Archeean d.

Coarse Grit.A Led of shale with included round masses up to

diameter.Greenish gnarled beds.

6 inches in longest

Fig. 7.-Section on shore S.w. 0/pier at Angleseaend ofGarth Ferry.

a/

{

a. Black flaggy shale with tracks such as are usually referred to annelids. CrushedCambrian junction with b.

b. Coarse grit, obscurely bedded. Base not seen.

206 EXCURSION TO NORTH WALES.

It was clear that the bedscommonly referred to underthe name quartzite in Holy­head Island are not true quart­zites, but are largely composedof felspar, their decompositionresulting in the productionof China-clay. They were, infact, only a development of thesandy bands in the gnarledschists; so we coined the namefelsi-quartzite for immediateuse until petrologists give usa better. We examined the

...z

'"-..o •~ "" '"oj ::l.. 0g:r:o

It is known that precisely the same kind of evidence as thatrecorded from the Bangor-Caernarvon district can be obtained in theisland of Anglesey, but we did not wait to examine it as we traversedthe Llanfaelog axis on our way to Holyhead. \Ve saw from thetrain the coarser porphyritic granitoid rock of Llaufaelog, whichthere forms part of the gneissic series, and, whether any of thegranitoid rocks associated with the Archrean may be intrusive into

the Archrean or may be only theextreme result of metamorphism,there is no suspicion roused inthat area that they are in­trusive into the Cambrian, andfor the principal object of ourenquiry the question is unim­portant, as other rocks of thegneissic series are abundantlyfound in the Cambrian Can.glomerate of Orsedd Person,Cefn-Ysgwydd, and other placesnorth-west of Llanfaelog." Sowe passed on to examine therelations of the quartzites tothe gnarled schists of Holy­head.

*See • Quart. Journ, Geol. Soc.' Vol. xxxviii, p. 25.

EXCURSION TO NORTH WALES. 207

line of junction, and it was clear that, unless there was a greatinversion of strata, the Holyhead felsi-quartzites passed up into thegnarled schists, as shown in the Fig. 8.

Such are the principal results of the investigation; but themembers, it can hardly be doubted, came away with the impressionthat for every good fact we carried away we left a hundred inter­esting problems unsolved.

On the 28th a large number of the members visited the Penrhynslate quarry, under the direction of the President. The manager,Mr. J. Evans, very kindly conducted them over the quarry, andpointed out the general succession and the" effects produced byseveral important faults. An interesting collection of specimensobtained from beds at different horizons was exhibited, and theeffects of cleavage on rocks composed of different materials ex­plained. Several of the members then travelled up the valley of NantFfrancon and examined the glacial strire so remarkably preservedalong' the river bed and along the sides of the valley. The ascend­ing succession of the Cambrian rocks was then followed to thewell-known Cruziana beds, and beautiful specimens of the latterwere collected. In traversing this section from west to east alongthe sides of Nant Ffrancon the following general order of the rocksseemed tolerably clear :-1, The purple and green slates of thequarry (Llanberis) ; 2, a thick series of grit and flaggy sandstones(Rarlech); 3, dark flaggy beds (Menevian ?) j 4, flags and sand­stones, with Cruziana (Lingula flags) j 5, dark flags (Tremadoc?);6, black slates and flags, with pisolitic iron ore and grit at base(Arenig).