THE EARTH BEFORE THE CAMBRIAN 389 



The Archeozoic Era 



Distribution of the Archaeozoic Rocks. — The rocks constituting 

 the Archaeozoic system are the oldest of which we at present have 

 any knowledge and, as far as known, underlie all the younger rocks 

 of the earth's crust. In regions which have been repeatedly uplifted 

 and eroded the Archaeozoic rocks are uncovered, and it is in such places 

 that they have been studied. In North America the greatest area of 

 Archaeozoic rocks lies in the eastern half of Canada, where they have 

 an area of about 2,000,000 square miles, forming an irregular mass 

 around Hudson Bay and extending south into Wisconsin and Minne- 

 sota. This is often designated as the " Laurentian shield." In the 

 Adirondacks of New York, in New England, and in a belt stretching 

 from Maryland south into Alabama (Piedmont Plateau) are crystal- 

 line rocks which are partly of Archaeozoic age. In the cores of the 

 mountains of the western half of the continent and in other isolated 

 patches they also appear at the surface. 



Our detailed knowledge of the Pre-Cambrian of North America 

 is largely confined to the region about the Great Lakes and the St. 

 Lawrence River. Here excellent and fresh exposures have been de- 

 veloped by glacial erosion, and the presence of valuable deposits of 

 copper, iron, nickel, cobalt, and silver has led to a careful study 

 of the region. 



Archaeozoic rocks apparently corresponding to the Archaeozoic of 

 North America occur in Scandinavia and other parts of Europe, over 

 a large area in Brazil, in central Africa, in China, in India, and else- 

 where, but the determination of the age of the crystalline rocks of 

 many regions is yet in cToubt. It has been roughly estimated that the 

 Pre-Cambrian rocks appear at the surface over one fifth of the land 

 area. The term " surface " is used to mean that the formation is 

 not covered by younger rock formations, although it may be hidden 

 in many places by soil or glacial deposits. 



The difficulty of any attempt to correlate the Archaeozoic rocks of 

 distant or isolated regions is obvious, since fossils are absent, and this 

 exact method of determining the age of rocks is consequently un- 

 available. Moreover the fact that the lithological character of the 

 rocks of a formation may vary greatly, even in short distances, makes 

 such characters of a formation an extremely uncertain criterion upon 

 which to base a correlation. However, since fossils are lacking, the 

 lithological character; superposition, and the degree of metamorphism 



