1846.] SEDGWICK ON THE FOSSIL SLATES OF N. WALES, ETC. 155 



east of the latter place. Near the upper limit of this group (which 

 I place on the parallel of a part of the Plynlimmon group and the 

 metalliferous slates of Cwm Ystwyth, &c.) are many specimens of 

 Graptolites foliaceus, and some very obscure traces of Corals and 

 Trilobites. 



4. A fine series of ridges composed of pale-coloured subcrystalline 

 slates, with beautiful cleavage-planes, and a structure which brings 

 the group into comparison with the Rhayader slates of the previous 

 section. Among these beds occurs also Graptolites ludensis. 



5. Beyond this group (following the line of traverse) we have a 

 series of earthy, shivery slates and flags — sometimes falling into in- 

 numerable small prisms — sometimes indurated into flags alternating 

 with bands of grit. The bands of grit are replaced by irregular 

 masses of coarse conglomerate, occasionally containing numerous 

 fossils. The series makes many undulations, and reaches the hills 

 which overhang the valleys and brows descending to the drainage 

 of the To wy. We now enter on the "Silurian System" (as laid 

 down on Sir R. I. Murchison's Map), which, as before stated, may 

 be divided into Lower Llandeilo flag — grits and slates generally of 

 a rather coarse structure, and rarely passing into the form of a con- 

 glomerate — and Upper Llandeilo flag, passing into the Upper Silu- 

 rian rocks. 



If I have interpreted these two South Welsh sections correctly, 

 they are in a good general accordance, however much they may 

 differ in certain subordinate details; and they convey, I trust, a 

 notion of the development of a considerable portion of the great 

 slate series of South Wales. They prove that there is a very great 

 thickness of slate rocks, in South Wales, inferior to the " Cambro- 

 Silurian group," which is also of great thickness. Yet all the groups 

 above enumerated are superior to the slates and porphyries of the 

 whole Cader Idris range, and I think also superior to the Bala 

 limestone ; but the sections in North Wales are so broken and difficult, 

 that on this last conclusion I cannot speak with perfect confidence. 

 The groups above noticed occupy, as already stated, a kind of trough 

 between the older rocks of Pembrokeshire and the chain of Cader 

 Idris, and are thrown into astonishing undulations. They also 

 exhibit many anomalous changes of strike, which may be partially 

 explained by the fact, that the greater part of their mass has been 

 exposed to two distinct and not contemporaneous movements of 

 elevation. The directions of the cleavage-planes are far more con- 

 stant than those of the beds, yet exhibit also many anomalies, which 

 I hope to point out in a subsequent communication. 



Upper Division of the " Silurian System " in South Wales. 



These rocks are finely developed in the parts of South Wales un- 

 der notice. It would be in vain for me to pretend to add anything 

 material to the beautiful details published in the great work of Sir 

 R. I. Murchison, and in the admirable memoirs and sections of the 

 Government Survey. I only pause to remark, 1st, That the develop- 



