796 



THE WONDERS OF GEOLOGY. Lect. VIII. 



In Cornwall schistose rocks (locally termed killas) 

 largely predominate, but they are referable to very distinct 

 geological epochs ; the newest belonging to the Devonian ; 

 the next in age to the Silurian ; while the most ancient, if 

 they can be separated from the latter, would rank with 

 those of Cumberland. But in the present state of the 

 question as to the most natural classification of the oldest 

 fossiliferous strata, it is impossible to arrange with precision 

 the slate rocks of particular regions. 



The following tabular arrangement explains the relative 

 position and characters of the subdivisions of the Schistose 

 strata of the Principality, adopted by Professor Sedgwick : — 



The Cambrian or Slate rocks of North Wales. 



(Commencing with the uppermost.) 



1. Plynlymmon rocks. — Greywacke and slate, with beds of conglo- 



merates. Thickness, several thousand yards. 



2. Bala limestone. — Dark limestone, associated with slate, contain- 



ing a few species of shells and corals. 



3. Snowdon rocks. — Slates, fine-grained, and of various shades of 



purple, blue, and green. Fine and coarse grauwacke and con- 

 glomerate. A few organic remains. Thickness, probably 

 several thousand yards. 



The upper dark-coloured schists contain a few corals and 

 fuci ; and Professor Phillips has discovered in the strata of 

 Snowdon two species of corals (Cyathophyllum), and six of 

 shells belonging to the family of JBrachiojwda. In the lower 

 series, fossils are of rare occurrence ; and in its few species 

 of fuci, corals, and shells, we see the last traces of organiza- 

 tion, and arrive at the extreme limits of the animal and 

 vegetable kingdoms of the ancient world, so far as are cog- 

 nizable by organic remains. 



The strata of Cumberland and the Lake districts, form a 

 system, which is thus defined by the same eminent philo- 

 sopher. 



