IIo 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 solid 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 clese 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 earlier (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 
