92 SILURIAN. 



ness is much diminished. They rest conformably upon the Upper 

 Llandovery rocks. Few fossils have been found in the beds, and 

 none of these are peculiar to them, all belonging to species that 

 occur in the Wenlock Series ; hence they are intimately associated 

 with that Series, and were at one time designated as Woolhope 

 Shales.^ 



Near Conway Castle, Graptolites have been found by Prof. Lap- 

 worth, including species of Motiogt-aptus, etc., and Corals have been 

 met with near Meifod and Llanfyllin. 



The Tarannon shales may be taken to include representatives of 

 the Pale Slates or Shales and the Graptolitic Mudstones of the 

 Lake District. 



According to Mr. W. Keeping central and west-central Wales 

 is made up almost entirely of a great series of imperfect slates and 

 greywackes belonging to the following group : — 



^ J. 1 • /-^ (2. Metalliferous-slate rroup. 2000 feet. 



Lardiranshire Group. < , ., . •.. •. /?■ j • 1 \ ^ ^r ^ 



" (I- Aberystwith grits (Sedgwick). 1000 leet. 



The rock-beds are remarkably folded and contorted, with frequent 

 inversions ; hence there is some doubt about the relative positions 

 of these strata. The included fossil remains, especially the 

 Graptolites, prove the Aberystwith Grits and Metalliferous slates 

 to belong to the same general geological horizon above the Bala 

 group and on the parallel of the Coniston INIudstones of the 

 Lake District. The grits contain JSlonograptus Sedgivickii, Phacops 

 elegans, etc. ; the slates have yielded Rasfriles peregrinus, etc. The 

 beds may be seen at Cefn Hendre, Devil's Bridge, Morben, near 

 IVIachynlleth, etc. Above the Cardiganshire group come the 

 Plynlimmon grits (1000 feet), forming a line of high country in the 

 centre of Wales, including Plynlimmon. These grits are probably 

 an arenaceous development of the Tarannon Shales, and the Cwm- 

 Elan conglomerates and Rhayader Pale Slates belong to the same 

 series. INIr. Keeping thinks that in the district there is evidence 

 of a passage upwards from the Bala to the Llandovery groups, 

 and from these into the Tarannon Shales and Denbighshire Grit 

 series.- Some plant-like remains occur in the Aberystwith Grits, 

 and under the name of NematoUtes Mr. Keeping has described 

 curious irregular branching structures of widespread occurrence. 

 (See p. 33.) The grits of Llangrannog are on the same horizon as 

 the Aberystwith Grits. ^ 



In North Wales, above the Corwen Grits, there are representa- 

 tives of the " Pale Slates," above which are the striped flaggy 

 slates of Penyglog, in which Prof. Hughes has found Afonograptus 

 priodon, Orthoceras prif}ia:vum, etc. These latter, together with 



^ Symonds, Records of the Rocks, p. 151. 



^ Q. J. xxxvii. 146. 



^ G. Mag. 18S2, p. 486. The terms Aberystwith and Plynlimmon groups 

 were used by Sedgwick, Q. J. iii. 151. See also J. E. Marr, Cambrian and 

 Silurian Rocks, p. 48. 



