THE HISTORY OF LIFE 185 



developed, they are estimated to attain a thickness of 

 more than 65,000 feet, or upwards of twelve miles." l 



The pre-Cambrian rocks are found in Great Britain, 

 Norway, Sweden, North America, India, China and 

 Australia. 



Lying upon or over the pre-Cambrian rocks, was 

 subsequently deposited a series of strata called by 

 geologists the Cambrian rocks, and forming the first 

 of the Primary formations. "Much interest neces- 

 sarily attaches to Cambrian fossils, for excepting the 

 few and obscure organic remains obtained from pre- 

 Cambrian strata, they are the oldest assemblage of 

 organisms yet known." . . . " One of the first reflec- 

 tions which they suggest is that they present far 

 too varied and highly organised a suite of organisms to 

 allow us for a moment to suppose that they indicate 

 the first fauna of our earth's surface. Unquestionably 

 they must have had a long series of ancestors, though 

 of these still earlier forms such slight traces have yet 

 been recovered." 2 



The Cambrian rocks in some places differ little from 

 the pre-Cambrian rocks below them ; but in other 

 places there is a marked break between them. Ripple 

 marks, formed by water as now seen on the sea-shore, 

 and sun-cracks are found in these rocks. 



Plants are scarcely at all found in these strata. No 

 vestige of land plants has been detected. Traces of 

 sea-weeds are believed to be found. 



Sponges, star-fish, jelly-fish (of which casts are found) 

 and crinoids, a group of echinoderms fixed to the 

 earth by a long stalk, existed when these rocks were 



i Idem, p. 697. 2 Idem, p. 720. 



