270 



LYELL'S ELEMENTS OF GEOLOGY. 



Cambrian Rocks. 



Cambrian Group. — Below the Silurian strata in the region 

 of the Cumberland lakes, in N. Wales, Cornwall, and other 

 parts of Great Britain, there is a vast thickness of stratified 

 rocks, for the most part slaty, and devoid of fossils. In some 

 few places a few jDrganic remains are detected specifically, and 

 some of them generically, distinct from those of the Silurian 

 period. These rocks have been called Cambrian by Professor 

 Sedgwick, because they are largely developed in N. Wales, 

 where they attain a thickness of several thousand yards. They 

 are chiefly formed of slaty sandstone and conglomerate, in the 

 midst of which is a limestone containing shells and corals, as at 

 Bala in Merionethshire. A slaty sandstone, forming the bottom 

 of the Cambrian system in Snowdon, contains shells of the 

 family Brachiopoda, and a few zoophytes.* 



In some of the slate rocks of Cornwall, referred by Professor 

 Sedgwick to the Cambrian group, cephalopoda of a very pecu- 

 liar structure, called Endosiphojiites, have been detected, a form 

 which appears not yet to have been observed in the Silurian 

 formation. The siphuncle in this shell is ventral, in which cha- 

 racter it differs both from ammonite, in which it is dorsal, and 

 from nautilus, in which it is central, or nearly central. 



Fig. 283. 



Endosiphonites carinatus, Ansted.t Cambrian strata, Cornwall. 



Although the Cambrian group can scarcely yet be said to be 

 established on the evidence of a distinct assemblage of fossils, 

 yet so great is the thickness of strata beneath the lowest of the 

 well-determined Silurian rocks, all of a date posterior to the 

 creation of organic beings, that we may reasonably expect to be 

 able to divide the primary fossiliferous strata into two groups. 



* Phillips's Geology, vol. i. p. 129. Lardner's Cyclop, vol. xcvii. 

 t Camb. Phil. Trans, vol. vi. pi. 8. fig. 2. 



