Murchison s Silurian System. 



233 



Bellerophon bilobatus, 

 Leptaena sericea. 



As these fossils also occur in the lower Silurian rocks, no 

 zoological division can yet be drawn between the lower 

 Silurian and upper Cambrian groups.* 



And as great lines of disturbance generally mark the 

 frontier of the two groups, the difficulty of defining their 

 common boundary is much increased. On the line of demar- 

 cation may occasionally be seen the various Silurian rocks, and 

 the quartzose slaty sandstone passing into a coarse grit, or 

 grey wacke of foreign geologists ; usually grey, but it is some- 

 times brownish and other ferruginous colours, and containing 

 casts of encrinites and corals, but in general void of fossils. 



The lower flags of the Silurian system (in those situations 

 in Wales where the junction is not interrupted by intrusive 

 trap, as on the western sides of the Grongar hills,) rest on 

 black schists, which overlie the Cambrian strata. These 

 schists are considered by Mr. Murchison and Professor 

 Sedgewicke, as the link that connects the Silurian and Cam- 

 brian systems. Mr. Murchison remarks, that the trilobites 

 and fossils of those beds bear so near a resemblance to those 

 described by Mr. Brongniart, from Angers in France, that 

 he regards the black flags in both places as the same.f 



Although the upper Cambrian rocks do contain fossils in 

 some places, in Caermarthanshire they do not. Even in 

 those places in which fossil remains have been found, it is 

 not in the fine slaty beds whose structure is so well calcu- 

 lated to preserve organic fossils that they occur, but in occa- 

 sional courses of hard grits or sandstones, which re-appear 

 at wide intervals in the schist. 



These greywacke rocks, or grey micaceous slaty sand- 

 stones and compact regenerated slates, often containing 

 fragments of older rocks in a quartzose cement, are gene- 



* Murchison 's Silurian System, p. 308. 

 t Silurian System, p. 358. 



