BASIS OF PRE-CAMBRIAN CORRELATION 19 



major periods in the pre-Cambrian history of Laurentia separated 

 by two critical epochs of diastrophism, with possibly a fourth period 

 represented by the Laurentian rocks at the base of the series. That 

 is to say we have three major periods in the pre-Cambrian succes- 

 sion separated by epochs of diastrophism, which diastrophism at each 

 epoch exhausted itself for the time. These are as follows : 



iKeweenawan-Athabasca 

 

 Upper Huronian or Animikie-Nastapoka 



( Middle Huronian 



Meso-Proterozoic j 



{ Lower Huronian 



I Keewatin 

 „ -p . \ (Intrusive contact) 



t.o-Froterozoic [Laurentian (embracing the original crust, if 



(^ any remains) 

 The lines drawn between the several subdivisions indicate unconformities, 

 the heavier lines indicating the major breaks referred to in the text. 



If we attempt to make a comparative study of the earlier conti- 

 nental evolution of North America and that of Asia, we note at the 

 outset that the Siberian nucleus is a portion of that northern Polar 

 region which comprises also Russia, Greenland, and Laurentia, 

 against which stress has been continuously exerted by the denser 

 masses of the more southern latitudes. As has been emphasized 

 by Suess, the Siberian nucleus has been undisturbed since a pre- 

 Cambrian date, and the same is essentially true of Laurentia also. 

 We find that in Asia there were in geological time great mediterranea 

 which, after they had been made the basins for the accumulation of 

 great thicknesses of sediment, were successively closed by great 

 thrusts from the south which folded up the sediments into mountain 

 ranges and then converted these into dry land. In Europe the Alpine 

 region was a marine strait in Cretaceous time, which was subsequently 

 converted in this way into a mountain range. 



In the North American continent, of which Laurentia forms a 

 part, there seems to have been a somewhat similar sequence in con- 

 tinental development. Thus the Appalachian Mountains and the 

 Cordilleran range of British Columbia represent ancient marine val- 

 leys or straits whose sediments are now folded into series of mountain 



