1867.] BRISTOW LOWER LIAS OF GLAMORGANSHIRE. 205 



of 25P in a direction 2b° west of south. The true position of these 

 beds between the ordinaiy Lias and the Carboniferous Limestone 

 is very clearly shown to the north ; and fossils are plentiful, Pijina, 

 Modiola, Ostrea Liassica, Lima, and Pecten (^ Suttonensis) being the 

 common shells. 



The three sections I have briefly noticed prove the true posi- 

 tion of the Sutton Stone relatively to the main mass of the Lias and 

 the Ehaetic series, and show that the conclusion " that the Sutton 

 series was probably slightly anterior in age to the Avicula-contorta 

 series " (Tawnej, loc. cit. p. 78) is untenable. 



It may not be out of place to remark here that the sand- 

 stones quarried for building and grindstones^ 1^ mile north of 

 Bridgend, on either side of the Eiver Ogmore, are not situated at 

 the base of the Keuper, as stated by Mr. Tawney, but are in the 

 upper part of the Ehaetic series, overlain by Lias crowded with the 

 characteristic Ostrea Liassica. The sequence of the beds is clearly 

 shown on the west side of the river at Angelton, where there is a 

 large quarry by the roadside, whence the stone was prociu^ed for 

 building the neighbouring County Asylum. 



The replacement by sandstones of the ordinary calcareous and 

 muddy sediments of which the Ehaetic Series is generally composed 

 indicates here a coast-line and shallow water. This is borne out by 

 the fact that it is in their most westerly extension in South Wales 

 (between Bridgend and Pyle) that this substitution of siliceous for 

 calcareous and argillaceous sediment has taken place ; and the Ehaetic 

 strata of the district now under notice represent the near- shore 

 deposits of a formation which, on the continent of Europe (in the 

 Ehaetian Alps, in Lombardy, Germany, and France), assumes a far 

 greater development than it does in this country. 



At a later date the Magnesian Conglomerates at the base of the 

 Lias indicate the shore- and shallow-water deposits of a country 

 which is now part of South "^"ales, and which, in part, was being gra- 

 dually submerged during the deposition of the Lias ; so that the 

 beds which rest on the irregular surface of the Carboniferous Lime- 

 stone are not necessarily the oldest part of the Liassic series, but of 

 different, though approximate, ages. 



Sir Henry De la Beche, in his masterly Essay* '• On the Formation 

 of the Eocks of South Wales and South-western England," well 

 describes the conditions under which these beds have been deposited, 

 and speaks of them as " a deposit, partly chemical, partly mechanical, 

 formed during depression t — occasionally perhaps kicluding low- 

 lands behind the shingle-beaches, the deposit of the time mingling 

 with the older rocks previously broken up on the surface by atmo- 

 spheric influences, and not sufficiently exposed to rounding by 

 attrition before they were enveloped by the matter of the new 

 deposit." He goes on to say that " in some localities we scarcely 



* Memoirs of the G-eological Suryey of Great Britain, 1856, vol. i. ]). 270. 



t See also " On the Denudation of South Wales and the adjacent Counties of 

 England," by Andrew C. Eamsay, F.Gr.S., in the Memoirs of the Greoloncal 

 Survey, vol. i, p. 297. 



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