GEOLOGY OF CENTRAL WALES. 



169 



slates) thus fall into line with the great pale-slate group of Britain 

 — the Tarannon group. 



N.E. Wales. — The Silurian rocks of Corwen, Llangollen, and the 

 Yale of Clwydd have been described by Prof. T. M'K Hughes * ; 

 and fossils have been collected from the Tarannon shales of the 

 Conway area by Mr. C. Lap worth, E.G.S. Those species which are 

 common to our Cardigan rocks are indicated in the Table ; but, 

 besides these, other species, characteristic of newer groups, are 

 found — namely jRetiolites, and Monogra/ptus priodon, colonus, and 

 galaensis. From these fossils Mr. Lapworth concludes that the 

 Conway beds " correspond with the earlier portion of the Gala beds " 

 of Scotland t- Thus the results of palaeontology correspond well 

 with the stratigraphical evidences, which show, in my opinion, that 

 the great mass of our Cardiganshire series is unrepresented in the 

 N.E. of Wales (as in the English border counties), or rather that 

 it is represented by the break which Professor Hughes has worked 

 out in that district at the base of his Corwen Grit. The black 

 bands in the Clwydd valley, from which Prof. Hughes obtained 

 Graptolites, do appear to lie in the parallel of the higher part of 

 our Metalliferous group. 



My friend Mr. J. E. Marr has just published the details of the 

 sections at Cerrig y Druidion, ~N. of Bala^;. He there finds beneath 

 the Tarannon shales the equivalent of the Graptolitic mudstones, 

 containing fossils of the same species as our Cardiganshire series, 

 but also including Monograptus colonus, which is characteristic 

 of much higher beds. The section appears to show a greater deve- 

 lopment of Graptolitic " mudstones " than in the Clwydd valley, and 

 so far to represent more fully our Cardiganshire group ; but here, 

 again, as at Corwen, the series appears to be incomplete, because 

 of the existence of the Silurian unconformity. 



General Summary. 



Central and West-central Wales is made up almost entirely of a 

 great series of imperfect slates and Greywackes belonging to our 

 Cardiganshire group, together with the overlying pale slates and 

 grits of Ehyader and Plynlimmon. The Cardiganshire group is sub- 

 divided into the (1) Aberystwyth Grits, and (2) Metalliferous Slates ; 

 and part of the underlying slates may, perhaps, hereafter be proved 

 to belong to the same group. Some minor subdivisions are also 

 distinguishable. The arenaceous rocks are not constant over large 

 areas, but die out both to north and south. 



The rock-beds are astonishingly folded into violent contortions, 

 with frequent inversions, especially in the Metalliferous series, so 

 as often to produce the misleading appearance of a regular and 

 continuous ascending series exceeding five miles in thickness. All 

 the important axes of elevation in the country have a common N". and 

 S. direction, two of the main folds being the Aberystwyth anticlinal 



* Quart. Jonrn. Geol. Soc vol. xxxiii. p. 207, vol. xxxv. p. 694. 



t Ann. Nat. Hist. 1880, vol. ix. p. 49. 



\ Quart. Journ. Geol. Soc. vol. xxxvi. p. 277. 



