GEOLOGY OF THE NORTH-WEST OF LAKE SUPERIOR. 219 



This land quite possibly existed in truly pre-glacial days. It re- 

 quires no great daring to imagine the time when ice was not. But 

 as soon as the globe had cooled so far as to admit of the existence of 

 water, ice would begin to show in winter at the poles, and slowly 

 extend from the i-egions within which the nights are four and three 

 months Ions to those in which they last but two and one. When it 

 reached from tlie north the contines of this primitive continent, gla- 

 ciers having meanwhile formed upon the mountain ranges, the ice 

 action, uniting with unchecked sub-aei'ial influences, would increase 

 in power and eflfect. So, age after age, the assault of the elements 

 would naturally degrade the whole extensive area, dispersing its 

 materials. Sooner or later barrier after barrier must go ; finally 

 (after several oscillations of level) even that one north of Lake 

 Superior — and so, we may be persuaded, the Great Laurentian Con- 

 tinent came to an end as such — by the effects of warm airs and 

 softening rains from the south and west, and the persistent attacks of 

 Polar cold from the north and east. 



Until the last great barrier of lofty hills had been overcome, the 

 general temperature south of it was probably quite mild, in spite of 

 latitude, but the breach having once been made, the waves of cold 

 would resistlessly pour thi-ough — and thus, perhaps, we may compre- 

 hend the extension of an ice age over a great adjoining area, and the 

 planing down of the heights to something near their monotony of to- 

 day. Thus too we can readily understand how the great plains north 

 of the height of land behind us were formed, very gradually sloping to 

 the Hudson's Bay ; how the James' Bay mudflats came to exist ; 

 how tl^e lands and islands of our Arctic Ocean came to be shaped as 

 the map shows them — with forms familiar to all who have observed 

 the glaciated regions of Muskoka and the northern shores of the upper 

 lakes. And if we wish to form an adequate idea of the vastness of these 

 forces, the length of time during which they operated, the height of 

 the old mountains, and the thickness of the strata worn away, we 

 have only to consider that from the ruins of this continent came the 

 tens of thousands of feet in thickness of the Cambrian, Silurian, 

 Devonian formations, and what others not, in great variety, ex- 

 tending over a quarter of million of square miles south of it, and 

 perhaps more yet to the north. 



It seems not unlikely that one chief breach o^ the rampart against 



