202 New York State Museum 



lava flows and five conglomerate beds have been found 

 in northwestern Minnesota and neighboring portions of 

 Wisconsin. The igneous outbursts became less frequent 

 toward the close of this period and sedimentary deposits 

 increased. It is in the lavas and conglomerates of this 

 time that the rich copper deposits of the Lake Superior 

 region are found. The Keweenawan series are regarded 

 by some geologists as belonging to the Cambrian. 



The Proterozoic era closed with a period of mountain 

 building which was followed by a prolonged interval of 

 erosion and peneplanation of the continent. The uncon- 

 formities separating the various divisions of the Protero- 

 zoic in the Lake Superior region are quite distinct and 

 are marked by erosion surfaces and basal conglomerates, 

 by differences in degree of metamorphism and amount 

 of volcanic eruption. Another region where Proterozoic 

 rocks have been identified is the Grand Canyon, Arizona, 

 where nearly 12,000 feet, mostly sandstone, are exposed. 

 They also form the cores of many of the western moun- 

 tain ranges, the thickest sediments occurring in western 

 Montana, eastern Idaho and British Columbia where the 

 combined sections show a thickness of 37,000 feet, mainly 

 sandstones and shales. Proterozoic rocks have been iden- 

 tified in the Black Hills of South Dakota and also in the 

 Piedmont Plateau of eastern North America with some 

 certainty. 



Life. Direct evidence from fossils for the existence 

 of life in the Archeozoic is very scanty. Sir William 

 Dawson's Eozoon canadense (Canadian dawn animal) is 

 no longer regarded as a fossil, but is believed to have its 

 origin in the metamorphism of calcareous deposits formed 

 through the agency of some organism, probably a marine 

 alga. Indirectly life is indicated by the vast amount of 



