EXCURSION TO SWINDON.SATUR1MY, SEPTEMBER 17TH, 1921.

REPORT BY C. H. GORE, F.G.S., C. P. CHATWIN, F .G.S., andJ. PRINGLE, F .G.S., Directors of the Excursion.

ON arriving at Swindon Station the party proceeded to theMuseum, where they were received by the Mayor of Swindon(Alderman E . Jones) and the Deputy-Mayor (Alderman A; E.Walters). Each member was provided with a geological mapof the district, drawn to the scale of five inches to the mile,and prepared for the occasion. With the map was a table ofzones (based mainly on Dr. H. Salfeld's scheme of classification)and their stratal equivalents both at Swindon and on the Dorsetcoast. One of the Directors then gave a brief account of thegeology of the district. It was mentioned that much light hadbeen thrown on the palreontology of the beds around Swindonsince the acquisition by the Geological Survey of a part ofthe collection of fossils formed by the late W. H. Hudleston,F.R.S., which had recently been presented to the Survey by MissCatherine A. Raisin, D.Sc.

In explaining the table of zones, and discussing the advantagesof adopting palreontological units for the subdivision of stratifiedrocks, it was remarked that the zonal method was more reliablethan subdivision based on lithological characters. Such charac­ters are frequently found to vary when strata are traced fromone area to another. Taking the Kimmeridge Clay as an example,it was pointed out that, while the lithological characters aregenerally uniform through most of the formation in England,confusion and error arose when these characters were relied on asguides to the limits of the series. In the type-sections nearKimmeridge Bay, the Kimmeridge Clay extends from the baseof the Pictonia zone up to the top of the zone of Perisphinctespallasianus, that is, from the top of the Corallian to the base ofthe " Portland Sands," as originally defined. At Abbotsbury,however, where the" Corallian" type of deposit continued for alonger period, the base of the Kimmeridge Clay was taken by theGeological Survey at a horizon within the Rasenia zones. AtSwindon, where the argillaceous deposits which characterizethe Kimmeridge formation had set in somewhat earlier, thebase of the Kimmeridge Clay was drawn by the Survey at ahorizon well below the top of the Corallian; whilst in someof the eastern counties it is hardly possible to separate these twoformations by their lithological characters, since a clay faciescontinues almost without interruption from the base of the Ox­ford Clay up to the top of the Kimmeridge Clay. On the Con­tinent the results are still more confusing. The greater portionof the Kimmeridge Clay is represented by an entirely differentset of sediments, and, as a result, the base of the Portlandian

EXCURSION TO SWINDON. IS3

is taken by French geologists at the top of the Aulacosiephanuszone, thus including in the higher formation the larger part of theKimmeridge Clay as developed in Kimmeridge Bay. It is obvi­ously undesirable for the term Kimmeridge Clay to be used todenote a mass of sands and limestones; but the stratigraphicalterms Kimmeridge, Portland, and similarly Purbeck, are derivedfrom English localities where these formations are typically-developed, and mark time periods during which a successionof geological events took place irrespective of sedimentation.Hence the French use of the term Portland for a series of depositslower than the Portland Beds of England is unjustifiable. How­'ever, by utilizing fossils which characterize successive levelswithin a formation, we obtain criteria independent of varyingsedimentation. The zonal method of classification is thusapplicable over practically the whole basin of deposit, andaffords a reliable means of correlating strata of diverse lithologicalfacies. In fact, the modern schemes of zonal classification arebut the natural outcome of the brilliant work of William Smith-on "Strata Identified by Organized Fossils," carried out withunrivalled insight a century ago.

The programme of the day was then explained, and the party,now augmented by several members of the North Wilts Camera'Club, set out for Messrs. Hill's Brickyard at King's Hill. Herethey examined a small exposure of clay in the Virgatites zoneof the Kimmeridge Clay; and among the fossils found wereProtocardia morinica (de Loriol), Camptonectes morini (deLoriol), Lucina minuscula, Blake, and a number of ammonitesshowing virgatotome type of ribbing. This brickyard is prac­tically the only exposure of Kimmeridge Clay in the district,'since Bazzard's (formerly Turner's) pits are now disused andwater-logged. Referring to Bazzard's lower pit, Mr. Pringlementioned that the basal clays of the Kimmeridge series hadevidently been exposed there, since Dr. Salfeld recorded thegenus Pictonia. The same basal clays were passed through in thedeep well made by the Great Western Railway at the westernend of their works. It is probable that from this horizon up tothe base of the" Portland Sands" there is at Swindon a completesequence of zones, although the thickness is only about onethird of that exposed in Kimmeridge Bay. Before the membersleft Messrs. Hill's brickyard, a demonstration of the processes-of brickmaking was given by Mr. E. Iles.

The party then proceeded to the Cemetery, where the upperbeds of the" Portland Sands" with doggers could be seen. HereMr. Gore had laid out for examination much material excavatedin the digging of graves. This came from the reddish-green'Calcareous sandstones overlying the "Portland Sands," andproved to be highly fossiliferous. A short time was devotedto collecting, and among the fossils obtained were Astarte

154 EXCURSION TO SWINDON.

polymorpha, Contejean, Modiola autissiodorensis, Cotteau, Proto­cardia morinica (de Loriol), and Perisphinctes easilecotteneis;Salfeld. Some of the clays and marls with Exogyra bruntrutana,overlying this bed, were also seen, and samples were obtainedof the lydite pebble-bed at the base.

On the way to the Okus Quarry the members saw the upperpart of the Swindon Clay with the overlying lydite-bed, in Pem­broke Street. After lunch much time was spent in collectingin the Okus Quarry. Here the lydite-bed at the base of thePortland Stone series is seen resting on the Swindon Clay underthe floor of this quarry, and above it a sandy limestone threefeet six inches in thickness, yielding the zone-fossil Perisphinctesgorei. This limestone is overlain by the" cockly bed," a marlyand sandy limestone about four feet in thickness, crowded withfossils, representing the zone of Perisphinctes pseudogigas,In addition to the zone-fossil it yields Protocardia dissimile (J.de C. Sowerby), Trigonia gibbosa, J. Sowerby, T. incurva, J. de C.Sowerby, Ostrea expansa, J. Sowerby, Synoyclonema lamellosa.(J. Sowerby), Perna bouchardi, Oppel, and Lucina portlandica,J. de C. Sowerby. A number of large ammonites from thisbed were laid aside for distribution among members of theparty. Next in order of succession were the buff and white false-­bedded sands with irregular lenticular layers of calcareous sand­stone, known as the Swindon Stone. No fossils were obtainedfrom these beds, although they were diligently searched by somemembers of the party. Before leaving this quarry the Directors­pointed out that beds still higher in the Portland Stone serieswere exposed on the eastern side of the Town Gardens, towardswhich the party then moved. The section (on the eastern sideof Quarry Road), is that referred to by Blake and H. B. Wood­ward, but is now largely obscured. Towards the northern end,however, the Swindon Sands, here exposed to a depth of IO feet,can be seen to be overlain by the following beds, in descendingorder :-.

ft. ins..Hard, white, chalky limestone with Lucina and

Cerithium portlandicum 3 0Thin seam of grey marl I

White-weathering, compact limestone, with smallgrains of quartz 1 6

Dark grey, clayey sand, with shell-fragments,weathering brown in upper part (Cerithiumportlandicum, &c.) 1 3

[Swindon Sands].These beds are undoubtedly of Portlandian age.The next layer, a greyish-brown, sandy marl, two feet six

inches in thickness, with pebbles at its base, rests on an irregularsurface of the underlying Portland, and the presence in it of

REPORT OF THE SESSION, 1921. 155

cyprids shows it to be of Purbeck age. It is followed by compact,pale-brown limestone with quartz-grains (three feet six inches)brown, cellular-weathering calcareous tufa, penetrated by numer­ous veins of calcite (three to four feet); thin-bedded compact,creamy limestone (one inch) ; and grey clay with a band of com­pact earthy limestone (two feet), and then by soil.

Although the present exposure in this cutting is very limitedthe unconformity between the Portland and the Purbeck isclearly seen. When the full length of the section was visiblehowever, and the beds of the Purbeck were seen not only to beinconstant but also to rest indifferently on varying beds of thePortland series, the succession was so puzzling that Blake re­marked" a more complicated and at the same time instructiveseries of sections I have never seen."

After leaving this section a brief halt was made near therailway-cutting at Lethbridge Road, where a further section ofthe Swindon Sands could be seen. The party, having thenviewed all the exposures to be visited, proceeded to the King'sArms Hotel, where they were entertained at tea by the Mayorand other members of the Corporation of Swindon.

REFERENCES.One-inch Geological Map, Old Series, Sheet 34.J. F. BLAKE.-Quart. Journ. Geol, Soc., vol. xxxvi., 1880, p. 203; Proc;

Geol. Assoc., vol. xii., 1891-2, p. 326.H. B. WOODWARD.-" The Jurassic Rocks of Britain" (Mem. Gcol, Survey).

vol. v., 1895; Ceol. Mag., 1888, p. 469.R. H. K. HALL.-" Geology of Swindan Hill." Trans. North Wilts. Field

and Camera Club, vol. ii., 1911, pp. 1-21.H. SALFELD.-Quart. f ourn, Geol, Soc., vol. lxix., 1913, p. 423; Neues

JaMb f. Min., Beilage Band xxxvii., 1913, p. 125.

REPORT OF THE SESSION, 1921.

By W. WRIGHT, F.G.S.

ORDINARY MEETING, Friday, January 7th, 1921.-W.Whitaker, B.A., F.R.S., F.G.S., President, in the Chair.

Miss H. A. Hippisley Barnes, de Barri Crawshay, HenryGeorge Dines, H. W. Grimes, George W. Lamplugh, JamesMaden, Richard Payne, Stanley Smith, Miss Kathleen Wardand Miss A. M. G. Yale, were elected members of the Association.

Messrs. A. S. Kennard and B. B. Woodward read a paperon "The Post-Pliocene Non-Marine Mollusca of the East ofEngland." A discussion followed in which Messrs. E. T. New­ton, G. W. Lamplugh, A. L. Leach, A. Wrigley, Reginald A.Smith, W. P. D. Stebbing and J. F. N. Green took part.

Messrs. G. S. Sweeting and Frank Gosslingwere elected auditorsof accounts for the year 1920.

ADDITIONAL MEETING, Friday, January zsst, 192I.-W.Whitaker. B.A" F.R.S., F.G.S., President, in the Chair.