Palaeozoic Rocks of Great Britain. Ill 



first including the Permian and Carboniferous systems ; the 

 second, theDevonian or Old Red Sandstone; and the third, the 

 Silurian and Cambrian systems. These rocks were charac- 

 terized generally by the entire absence of Mammalia, and even 

 of reptiles in their lower division ; and by the presence of 

 peculiar groups of shells (Orthocerata and Goniatites), 

 crustaceans (TWfofo'tes), and corals (e.g. Graptolites). Very 

 few specific forms ranged from one division of this system to 

 another ; but they had great general resemblance. A few 

 corals ranged from the Bala limestone to the Devonian, and 

 one (Favorites Gothlandia) even to the lower beds of carbon- 

 iferous limestone ; Terebratula reticularis was found in the 

 Silurian and Devonian ; an&Leptaena depressa from the Bala 

 limestone to the Carboniferous. Prof. Sedgwick then called 

 attention to the grounds for separating the Cambrian and Si- 

 lurian systems, which he said he had always maintained to be 

 distinct. He had commenced his observations in the Cumber- 

 land Hills, of which a section was exhibited, shewing the fol- 

 lowing successions of rocks : — 1. Skiddaw, slate, usually with- 

 out fossils, but containing graptolites in one locality ; 2. Con- 

 iston limestone, abounding in fossils ; 3. Coniston flagstone 

 and grit. The order of succession of the beds above these 

 was difficult of determination in the Lake district. He had 

 next investigated the structure of North Wales, between the 

 Menai and the Berwyns, and had established the existence of 

 a great system of rocks comparable to those of the Lake dis- 

 trict, and had given to them the name of the Cambrian system. 

 Meanwhile, Sir B. Murchison had discovered in " Siluria" 

 tracts exhibiting the whole upper series, equivalent to the 

 beds above the Coniston grit. And having made good sec- 

 tions of the strata in ascending series, from the Llandelio flags 

 to the Old Bed Sandstone, and given names to these rocks 

 which were now generally adopted, this country had become 

 the type to which all others were referred for comparison, 

 because in it the order of succession was clearly made out. 

 It then became a question what was the boundary line be- 

 tween the Cambrian and Silurian systems % Sir B. Murchi- 

 son had made the Llandeilo flags the base of his system, and 

 considered the whole country westward of it to be Cambrian. 



