THE EARTH BEFORE THE CAMBRIAN 



397 



I 



Fig. 371.— 

 An appendage 

 of a crustacean 

 {Beltina danai) 

 from the Prot- 

 erozoic. This 

 is one of the 

 oldest known 

 fossils. (After 

 Walcott.) 



existence of shell-bearing animals, such as are now forming the cal- 

 careous ooze and shell deposits of the ocean bottom. Graphite and 

 black shales are suggestive of plant remains. The great deposits of 

 iron ore are thought to indicate the existence of life, 

 since organic matter seems necessary to have furnished 

 the carbon dioxide by means of which the insoluble 

 iron minerals were decomposed, and as soluble iron 

 carbonates were carried away and redeposited where 

 the further movement of the underground water was 

 prevented. It is possible, however, that decomposing 

 organic matter may not have been essential to this 

 process. 



Direct evidence is furnished by a few fossils that 

 have been found in the Proterozoic rocks of the 

 Grand Canyon of the Colorado in Arizona, and in 

 rocks of this age in Montana and Ontario. The 

 known animal life consists of several species of worms, 

 a large crustacean (Fig. 371), a sponge-like fossil (Atikokania), 1 some 

 of which are 15 inches in diameter, and a brachiopod. Abundant 

 fossils of a calcareous alga (Fig. 372), individuals of which are more 

 than two feet in diameter, form layers of limestone three feet thick. 

 It is probable that when all parts of the world become geologically 



better known, fossils will be dis- 

 covered in Proterozoic formations 

 as distinctive in character as those of 

 the Cambrian and overlying systems. 

 Duration. — The fossils of the Prot- 

 erozoic, though few and fragmentary, 

 show that some forms of life were well 

 up in the scale of life. Crustaceans, 

 worms, and brachiopods (p. 414) are so 

 high in the scale as to force the con- 

 clusion that life had been in existence 

 many millions of years prior to this 

 time. Moreover, judging from the 

 extreme slowness with which evolu- 

 tional changes take place, the great differentiation in the life proves 

 a great antiquity. When this evidence, even though theoretical, is 

 taken in connection with the great thickness of the sediments and 

 1 Atikokania is probably not a sponge but a calcareous alga. 



Fig. 372. — Hemispherical bodies 

 believed to have been formed by blue- 

 green algae. Proterozoic, Montana. 



