THE PROTEROZOIC ERA 329 



13,000 feet thick, while the Keweenawan lava flows and sand- 

 stones may have a thickness of 35,000 feet. If to the time 

 required for the making of these rocks we add the long lapses 

 of time represented by the various unconformities, it becomes 

 evident that the Proterozoic era was one of the longest. By 

 comparing the thicknesses of younger systems it has been esti- 

 mated that it may have been as long as all the subsequent 

 periods combined. 



LIFE IN THE PROTEROZOIC ERA 



Evidence from the sediments. In the Archaeozoic era 

 living things are believed to have been present, but the evi- 

 dence of their existence is somewhat indirect, for no fossils 

 have been found. Again, in the Proterozoic systems of rocks, 

 we find limestones, this time in thick layers, which may have 

 been made partly of the shells of minute animals, just as 

 more recent limestones have been. 



It is well known that coal beds have been derived from 

 compressed masses of the vegetation which accumulates in 

 swamps. Coaly layers and beds of graphite among the Pro- 

 terozoic rocks probably had the same origin. Still other facts 

 make it almost certain that both plants and animals were 

 abundant throughout the era. 



Fossils very rare. Among the younger strata of the Pro- 

 terozoic group a few poorly preserved fossils have been dis- 

 covered. They are the remains of animals, and among them 

 are forms which seem to belong to the brachiopods (p. 298) 

 and the crustaceans (p. 300). 



The crustacean group is one of the most advanced of all 

 the invertebrates, and it is therefore somewhat surprising 

 that it should have appeared so early. Few though they are, 

 these fossils justify us in believing that the living world had 

 been in existence for untold ages before the strata which 

 contain them were deposited, and that the slow changes of 

 evolution had already produced some types not altogether 

 unlike those of modern times. 



