BASIS OF PRE-CAMBRIAN CORRELATION TOS 
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: 
( Keweenawan-Athabasca 
INGO-Proterozoic a4... fs 52: 
| Upper Huronian or Animikie-Nastapoka 
\ Middle Huronian 
IMieso=ProteroZOlG.. 22. ...3.- 4 
{ Lower Huronian 
( Keewatin 
j (Intrusive contact) 
| 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. 
WOsPrOterOZOIG: aces eee e. ane 
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 fange. 
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 
