34 JOURNAL AND PROCEEDINGS. 



gone forward rapidly. The atmosphere, the rains and the hot waterg 

 became effective agencies in altering the physical features of the 

 earth by erosion, and the fundamental rocks began to be covered by 

 the sedimentaries. But the internal forces were active yet and for 

 ages after ; the mountain-making folding continued, and great masses 

 of igneous rocks were intruded into the cooling crust or extruded 

 upon it. The waters of the sea grew in volume, the Archaean high- 

 lands subsided, and once or twice in their history, if not oftener, 

 they were over a very large extent submerged. In that sea the 

 Huronian rocks — possibly a portion of the Laurentian also, and the 

 foliated members of it certainly if they are sedimentary — were laid 

 down, but we have no data for calculating their mass. The Huron- 

 ians extended over large areas to the north and south, much of 

 which is hidden by overlying deposits ; in the typical region north of 

 lake Huron their thickness was computed by Murray to be 18,000 

 feet, and their aggregate thickness as originally laid down may have 

 been not less than 40,000 or 50,000 feet. At two successive periods 

 in their history the rocks of this great system were folded and tilted 

 into mountain forms, followed by two long periods of active erosion 

 during which the denudation was deep enough to remove the entire 

 series in places, and wear the mountains down to stumps. How far 

 if at all, glacial agencies operated in this cutting down and carrying 

 away of Huronian material to construct new systems, there is no 

 means of determining ; but there is nothing improbable in the sup- 

 position that they were as active in those early ages of the earth as 

 they have been in the later period, the record of which the ice has 

 so left written upon the face of the rocks that we may read it. 



Following the Huronian system by the classification of the 

 Canadian geologists, there come next in order the formations of the 

 Cambrian system, embracing the Animikie, Nipigon and Potsdam, 

 with an aggregate thickness of 54,000 feet according to some meas- 

 urements, and of 63,000 feet according to others. The Nipigon 

 alone has a thicknees computed at 50,000 feet, composed almost 

 wholly of gabbros, diabases, amygdaloids and lavas ejected through 

 fissure and crater during a long period of volcanic activity, and re- 

 sulting in the great east and west synclinal which forms the basin of 

 lake Superior. 



After the Cambrian rocks came those of the Silurian systeni 



