Vol. 67.] THE CARBONIFEROUS SUCCESSION IN GOWEB. 493 



and fine-grained matrix in varying proportions. They are argil- 

 laceous and some, at least, are dolomitized — the rhombohedra 

 being, as a rule, minute. The shale-bands and partings are 

 more or less calcareous and, though abundant, generally less 

 than 2 feet thick. The chert, largely made up of sponge-spicules, 

 is abundant in some beds, either as black flinty nodules, more 

 or less sharply defined from the matrix, or diffused and difficult 

 to detect except under the microscope; it also occurs as beekite, 

 replacing fossils, in some beds apparently in this form alone. 



'Rottenstones': relations to the 'Black Lias.' — The 

 ' black lias ' characters are retained only where the rocks are 

 unweathered. In places the beds, both limestones and shales, 

 have been completely altered by decalcification and oxidation to 

 light-grey or yellow-brown, so-called ' rottenstones ' l and clays. 

 The ' rottenstones ' are either friable and largely argillaceous, or 

 brittle, like porous earthenware, and partly siliceous — the latter 

 evidently representing diffusely-cherty limestones. Many are 

 cavernous with moulds of unsilicified fossils, and some display in 

 sharp relief many details, hidden in unweathered material, of the 

 structure of silicified fossils. Wavellite 2 occurs in cavities and 

 joint-planes in the higher ' rottenstones,' as in the overlying 

 radiolarian cherts. 



The ' rottenstones' and clays differ so materially at first sight 

 from 'black lias,' that their equivalence is masked; but the view 

 put forward by Mr. Tiddeman, 3 that they have been derived by 

 process of ordinary weathering from such beds, is confirmed by 

 the relations of the two types exposed in the northern quarry 

 at Colts Hill. There, D l2 , which is carried by the dip from 

 top to bottom of the quarry, is followed by ' rottenstones ' and 

 clays with cherts near the top, but by ' black lias ' with the 

 same fauna at the bottom. Other palaeontological evidence, 

 also, points to the equivalence of the two types : the fauna of the 

 Bishopston * rottenstones ' being approximately the same as that 

 of the Oystermouth 'black lias ' (p. 551). 



An explanation of the extensive character of the weathering 

 which the ' rottenstones ' have undergone is probably to be 

 sought in the fact that the latter, so far as known, are situated 

 as at Bishopston, that is, at or near the top of the partly 

 dissected plateau which constitutes the whole of this part of 

 Gower ; whereas ' black lias ' is found at lower levels, as at 

 Oystermouth. It is believed that the conditions of minimal 

 run-off and denudation which characterize the top of the plateau 

 have lasted long enough to allow the rocks there to be completely 

 weathered to a considerable depth. In the valleys, where denu- 

 dation is rapid, the outcrops at lower levels are unweathered. 



1 None of the beds have been worked for rottenstone ; they are not so fine- 

 grained as the material of that name dug for polishing-powder on the 'North 

 Crop.' 



2 First noticed by Logan : see Mem. Geol. Surv. vol. i (1846) p. 134. 



3 Swansea Memoir, pp. 25-26. 



2m2 



