442 HISTORICAL GEOLOGY. 



The subdivision of Archaean time into Azoic and Archseozoic, here used, is the same 

 as that of the edition of 1874, except that ArehiBozoic is substituted for Eozoic. The limit- 

 ing temperature of Arohfeozoic time is doubtful for several reasons, and especially because 

 of the uncertainty as to the destructive excess of carbonic acid in the air and waters, 

 and, therefore, as to the possibility of the existence of life. 



There is reason to believe that during the progressing consolidation, and long after- 

 ward, when the heat was too great for the existence of limestones, the lime now in the 

 limestones of the globe was, to a large extent, combined with silica, making silicates and 

 especially the lime-soda feldspars, labradorite and oligoclase, the soda being that now in 

 the ocean's waters — minerals that may be made artificially by fusing together the 

 ingredients ; and, consequently, that rocks of the basalt and dioryte types, which con- 

 tain these feldspars, were among the most common. Pyroxene was present through the 

 whole era, but hornblende only in the later part ; for pyroxene is easily made at the 

 high temperature of fusion, but hornblende only under aqueo-igneous action at the lower 

 temperature of 800° to 1000° F. The lime silicates would have used up a large part 

 of what is now free silica or quartz, and hence the igneous rocks would have been, to a 

 great extent, without quartz, and, in this respect, like the most of those that come up 

 from the earth's depth through volcanic eruptions. In fact, most volcanic rocks are por- 

 tions of the Archaean mass constituting the earth's interior. Such being the prevailing 

 rocks of the crust, the sedimentary beds would have been largely of like constituents. 



On the condition of the primeval globe, see further Ebelmen, 1855 ; Bischoff, 1863 ; 

 T. S. Hunt, 1867, 1880. On subdivisions of Archaean time, D., 1892. 



NORTH AMERICA. 

 DISTRIBUTION OF ARCH.ffiAN AREAS. 



Archaean areas, or those whose surface rocks are of Archaean age, and 

 which indicate, therefore, the probable position of the dry land at the close 

 of Archaean time, have their widest distribution in the more northern por- 

 tions of the continents of North America, South America, and Europe. 



In North America they cover a very large area, situated mostly north of 

 the Great Lakes and the St. Lawrence River, which is approximately 

 V-shaped in its southern part, as shown in the accompanying map. The 

 white areas on the map represent the probably emerged land over the great 

 Archaean continental sea. The great northern area has been estimated to 

 contain more than 2,000,000 square miles. From the region of the Great 

 Lakes a broad arm stretches northeastward to Labrador and beyond, and 

 another, 2000 miles long, northwestward to the Arctic shores. Hudson 

 Bay, 800 miles from north to south and 600 in greatest breadth lies between 

 the arms of the V. The eastern arm of this early dry land of North 

 America has a course nearly parallel to the existing eastern coast-line of 

 the continent, and the western as nearly to the mean direction of the 

 western coast-line. The map is on Mercator's projection. The course of 

 the Mississippi River and the outlines of lakes are inserted merely to mark 

 positions. The Archaean area extends south of British America into northern 

 New York, the Adirondack region being a portion of it, and also south of 

 Lake Superior into northern Michigan and Wisconsin. 



