148 



ARCH.EAX TIME. 



IV. A fourth era, commencing with the beginning of life on the 

 globe, — which beginning was possible, judging from known facts, 

 when the temperature of the waters had cooled down at least to 

 200° F. It has been supposed that all the Archaean rocks open to 

 view over the earth's surface are those of this last era. But more in- 

 vestigation is required, before it can be regarded as an established fact 

 that none of earlier time are open to investigation. From these rocks 

 in America, two principal periods have been indicated, with other sub- 

 divisions. 



I. Distribution of Archaean Regions. 



The Archaean rocks of North America are mostly crystalline or 

 metamorphic rocks, and their beds stand at all angles, owing to the 

 uplifting and flexing which they have undergone. "Where the Silurian 

 strata overlie them, the two are unconformable, the latter being often 

 spread out in horizontal beds over the upturned edges of the Archaean 

 rocks. This position of these rocks is illustrated in the following cuts. 

 In each, the Archaean, numbered 1, in its usual disturbed condition, is 

 overlaid nearly horizontally by the Silurian beds of the Potsdam and 

 other periods, numbered 2 to 4 ; 2 being the Potsdam sandstone, 3 

 the Calciferous sandrock, 4 a the Trenton limestone, 4 b the Utica 

 shale. 



Fig. 203. Fig. 204. 



Fig. 205. 



Fig. 203, by Emmons, from Essex County, N. Y. ; 1 is hypersthene rock, or hypersthenyte. — Fig; 

 204, by Owen, from Black River, south of Lake Superior ; 1 is a granytio rock, 1 a, chloritic and 

 ferruginous slates. — Fig. 205, by Logan, from the south side of the St. Lawrence in Canada, 

 between Cascade Point and St. Louis Rapids ; 1, gneiss. 



This formation in North America was first distinctly recognized in its true importance 

 in the Report of Foster and Whitney on the Lake Superior region, in which it was 

 named the Azoic system. Dawson, after his announcement of the animal nature of the 

 Eozoon, suggested the name Eozoic (from >jw?, dawn, and Oot, life)- As the supposed 

 Eozoon may be of mineral nature, its use here is objectionable. 



The areas of the earth's crust over which the Archaean rocks are 

 now exposed are, — 



1. Those which have always remained uncovered. 



2. Those which have been covered by later strata, hut from which 

 these superimposed beds have been simply washed away, without much 

 disturbance. 



