360 



BEDS, CONCRETIONS, AND CLEAVAGE OF SLATY ROCKS. 



casionally affecting a slaty cleavage, and frequently appearing as if dipping to the north-west, when 

 the dip of the beds is to the south-east at 70° or 80°. Good examples of this are observable to the 

 south-east of the Sugar Loaf, near the spot where these lower strata pass into the Silurian System. 



The separation of the Lower Silurian rocks from the Upper Cambrian, has been generally effected, 

 by assigning to the former those beds which contain fossils, and to the latter those which do not. 

 For although animal remains occur in the Cambrian strata in many other parts of England and 

 Wales, nature has here afforded us no such evidences, since the incoherent schists near the base 

 of the Silurian System, and those which extend over so large a portion of the region of slaty 

 Cambrian rocks, are lithologically i?isejjarable. As we ascend in the higher and more arid regions 

 of the north-west, ribs of grit and sandstone begin to alternate with these slaty schists, and finally 

 the beds of schist becoming harder, have glossy laminae, are penetrated by thin veins of white 

 quartz, and put on more the characters of slate. Still older slaty strata, lying still further to the 

 north-west, contain thick courses and concretions of quartzose grit, and in these the dip is often 

 reversed to the north-west. The fragments of slate included in these coarse grits are sometimes 

 indented by the surfaces of small quartz pebbles 1 . In their range to the west of Llandovery 

 (PI. 34. fig. 2.), these rocks are here and there 

 diversified by concretionary masses of grit, some of 

 which have been recently exposed, by cutting a new 

 road from Llandovery to Llampeter in the gorge of 

 Cwm-dvvr 2 , north-west of Llanwrda. In some parts 

 these concretions are perfectly isolated in the slaty 

 mass, in others they run into contorted and imperfectly formed strata, in which obscure lines 

 of bedding are perceptible, dipping to the south-east and north-west, whilst the predominant lines 

 of slaty cleavage are invariably inclined to the north-west, the latter being what most observers 

 would take to be the true lines of stratification. The section (f. 2. PI. 34.) illustrates these ex- 

 amples, and affords an additional confirmation of the truth of the doctrine of slaty cleavage taught 

 by Professor Sedgwick. (See p. 352.) Either, however, the crystalline slaty impress (as is indeed 

 most probable), has not here been communicated in so decided a manner as in other parts of Wales ; 

 or the quartzose masses have been gathered together by concretionary action, subsequent to the 

 operation which produced the slaty structure, for the parallel planes of the slaty beds often terminate 

 abruptly against the edges of the concretions 3 . The schistose beds at their points of contact with 

 these concretions are more sandy and micaceous than usual, with ferruginous discolorations pro- 

 ceeding from the. decomposition of iron pyrites. This ingredient (the curse of most countries where 



1 It is from the summit of one of these wild mountains, Esgair davydd, that the large sketch, facing p. 346, 

 /Was drawn, combining, in one view, all the formations from the Old Red Sandstone to the greywacke slates 



inclusive. 



2 A different Cwm-dwr from that in which the Upper Silurian rocks are so well developed. 



3 This phenomenon is precisely in accordance with what I have recently observed in parts of the slaty 

 schistose system of Ilfracombe in North Devon, when in company with Professor Sedgwick. There the beds 

 of sandstone predominate; and the slaty cleavage, after passing through the schist, stops suddenly at the bed 

 of sandstone, thus : 



