402 



THE CHANGING GENERATIONS 



that may once have been present must have been destroyed by the heat 

 and pressure of diastrophism and vulcanism. 



One thing, however, suggests the presence of life during the Archeozoic. 

 In New York, Ontario, and Quebec there is a vast body of metamorphosed 

 limestones, shales and sandstones known as the Grenville series. The 

 limestones of later periods were made largely through the agency of 

 organisms, but since lime carbonate can also be precipitated chemically 

 from sea water, this is not conclusive evidence that life existed in Grenville 

 times. The deposits also contain flakes and masses of graphite, a meta- 

 morphic form of free carbon. The only known source of carbon in later 

 rocks is the bodies of plants and animals. If this was the source of the 

 Archeozoic graphite, we must infer that life was abundant in the seas of 



Fig. 26.1. The general relations of Archeozoic, Proterozoic, and Cambrian rocks and the 

 major unconformities between them shown in diagram. (Redrawn after Chamberlin and 

 Salisbury, Geology, courtesy Henry Holt and Company, Inc.) 



that time, for it has been estimated that there is more carbon in these 

 rocks than in all the Appalachian coal beds. 



The Archeozoic era closed with the first of the great "revolutions"; 

 all the continents were uplifted, lofty mountains were folded up, and a 

 long period of erosion followed, in which all the lands were worn low. 

 When a new cycle of deposition began in the Proterozoic era, conditions 

 had changed; igneous activity had lessened, and sedimentary rocks 

 formed in interior seas were predominant. In most regions they have been 

 metamorphosed, but in certain areas they remained unchanged and have 

 come down to us as the oldest strata in which fossils could have been 

 preserved. Many geologists have sought in vain for traces of life in these 

 rocks. 



Some fossils, however, have been found. In the Big Belt Mountains 

 of Montana and in Canada there are extensive limestone deposits com- 

 posed of rounded masses similar to those produced by certain lime- 

 secreting algae of the present time. Much doubt has existed as to whether 

 these were of organic or inorganic origin, but most authorities now agree 

 that they were precipitated by algae, probably of the blue-green group. 

 Other Proterozoic fossils that have been identified with fair certainty 

 are those of bacteria, brown algae (seaweeds), a jellyfish, casts of annelid 

 worm burrows, and what are probably silicious sponge spicules. Reports 



