4 o8 HISTORICAL GEOLOGY 



is often difficult to draw a line between them. In fact, it has been 

 suggested (Ulrich) x that the former dividing line be disregarded and 

 the Upper Cambrian and a portion of the Lower Ordovician consti- 

 tute a separate system called the Ozarkian. 



Other Continents. — The Cambrian system is represented in Wales 

 (20,000 feet) and Brittany by formations of great thickness. It also 

 occurs in Scandinavia, Russia, Siberia, China, India, Australia, Ar- 

 gentina, and other parts of the world. 



There appears to have been a land connection between North 

 America and Europe, or at least a chain of islands separated by shal- 

 low water, in the Cambrian. This is shown by the strong resemblance 

 of the fossils of this age in eastern North America and Europe, a 

 similarity which would not have been possible had the animals in- 

 habiting the shallow waters of the shores been unable to migrate from 

 one continent to the other. 



Life of the Cambrian 



The richness of the life of the Cambrian is in marked contrast to 

 that of the Pre-Cambrian, although the presence of worm trails and a 

 highly developed crustacean {Beltina danai, Fig. 371, p. 397) in the 

 latter indicates that the life of that ancient time comprised many 

 forms of invertebrates. However, so few specimens have been found 

 and so obscure is the evidence that little more can be said at present 

 than that the facts indicate that life was well developed before 

 Cambrian times began. 



The apparently abrupt appearance 2 of the earliest known Cambrian 

 fauna is probably to be explained by the absence on our present land 

 areas of the sediments and fossils of the period between the Protero- 

 zoic and the Cambrian. This resulted from the continental area's 

 being above sea level during the development of the unknown ances- 

 try of the Cambrian fauna, 3 and consequently the sediments of that 

 time are now covered by the sea and cannot be studied. 



The indirect evidence of the existence of life long antedating the 

 Cambrian is even stronger than the direct. A comparison of the life 

 of the Cambrian with that of to-day shows that of the eight branches 



1 Ulrich, E. O., — Revision of the Paleozoic Systems: Bull. Geol. Soc. America, Vol. 22, 

 191 1, pp. 281-680. 



2 Walcott, C. D., — Abrupt Appearance of the Cambrian Fauna on the North American Conti- 

 nent: Smithsonian \lis< . Coll., Vol. 57, No. 1, 1910, pp. 1-16. 



3 The term fauna means the total animal life of a certain region or period. 



