20 FRANK D. ADAMS 



ranges. The thrusts which closed up these mediterranea and developed 

 mountain ranges from them, were exerted in a northeasterly direction 

 against the southwestern part of the continent, and in a northwesterly 

 direction against the southeastern border of the continent, so 

 that the folds are parallel to the margin of the present continent 

 of Laurentia. If we inquire whether similar long, narrow, belt- 

 like mediterranea existed in Laurentia in pre-Cambrian times, 

 the answer seems to be in the negative. The surface of the con- 

 tinent seems rather to have had upon it at intervals throughout 

 geological time a succession of large, irregular-shaped bodies of water, 

 somewhat resembling the present Hudson's Bay, in which, however, 

 screat thicknesses of sediment were accumulated. 



The sediments deposited in these bodies of water in Keewatin, 

 Grenville, and the Earlier Huronian times, were folded up into moun- 

 tain ranges crossing the southern portion of the protaxis in a north- 

 easterly and southwesterly direction, coinciding with the course of 

 the Appalachian folding. 



The intense diastrophism which brought to a close the Eo-Protero- 

 zoic and again the Meso-Proterozoic time was exerted apparently 

 as far north as the middle of Labrador and the southern portion of 

 Hudson's Bay. 



In the later pre-Cambrian mediterranea the Nastapoka-Animikie 

 series and the Athabasca-Keweenawan series were deposited. The 

 almost entire absence of orogenic movement at the close of this time, 

 combined with the great extent and comparatively unaltered character 

 of the rocks, makes the break at the base of the Nastapoka-Animikie 

 series probably the most pronounced in the whole pre-Cambrian 

 succession in Laurentia. Thousands of square miles of practically 

 flat-lying sediments overlie remnants of a highly folded and meta- 

 morphosed antecedent series. 



We thus have two major breaks in the pre-Cambrian succession, 

 each marked by an epoch of diastrophism which exhausted itself for 

 the time. 



An identical series of two major breaks in the Proterozoic suc- 

 cession, marked by epochs of pronounced diastrophism which in 

 each case exhausted itself, is found in the Asiatic portion of the 

 nucleus. 



