14 FRANK D. ADAMS 



lower surfaces of the invaded sediments. Everywhere over thousands 

 of square miles these ancient sedimentary rocks can be seen to have 

 floated on the granite magma or to have been sunk into it and to have 

 been cut to pieces by apophyses of it. That these movements were, 

 in many cases at least, very slow, is shown by the fact that a study of 

 the primary gneissic structure displayed by the bathyliths demonstrates 

 that the upward movement of the latter began before crystallization 

 had set in and continued while the magma was slowly filling with the 

 products of crystallization and until it finally froze into a sohd rock. 

 This epoch of diastrophism, resulting in the elevation of great tracts 

 of country, brings to a close the first clearly recognizable chapter in 

 the history of Laurentia. 



After prolonged and profound denudation the sea again transgressed 

 upon the continent of Laurentia and in this sea were laid down the 

 strata of the earlier Huronian time. The sea at this time passed over 

 what is now the region of the Great Lakes and extended at least as 

 far north as Lake Mistassini and as far west of the head of Lake 

 Winnipeg. Locally it evidently extended as far inland as the latitude 

 of the northern end of Hudson Bay. Within this earlier Huronian 

 time there was, following the deposition of the Lower Huronian, a 

 period of subordinate elevation and depression in the district of the 

 Great Lakes marked by the deposition of the Middle Huronian. At 

 the close of this period of deposition, there was again an epoch of 

 widely extended diastrophism due to a thrust exerted upon the south- 

 ern portion of the continent from the ocean bed to the southeast and 

 resulting in the widespread folding of the sediments which had been 

 deposited over the southern portion of the protaxis, into a series of 

 mountain ranges running in a northeasterly to southwesterly direction, 

 with accompanying metamorphism of the folded strata and deep- 

 seated intrusion of vast amounts of igneous rock. It may be that the 

 great body of sediments forming the Grenville series really belongs 

 to this rather than to the earlier Keewatin period, but be that as it 

 may, these great orogenic movements which took place at the close 

 of the earher (Lower and Middle) Huronian time, brought to a close 

 the second great chapter in the pre-Cambrian history of Laurentia. 



There then followed a period of deep and long-continued erosion, 

 during which the Lower and Middle Huronian and the underlying 



