142 Transactions of the Canadian Institute. [Vol. VII. 



which the northern portion of this continent is supposed to have, at 

 different times, stood at different but undetermined elevations above the 

 then sea-level, the rocks of this area were exposed to the atmospheric 

 agencies of disintegration and degradation. The result was the develop- 

 ment of a topographic system whose remnants, though partly obscured 

 by the deposits of the Pleistocene epoch, are still recognizable. 



Extensive climatic changes, by some supposed to be the product of, 

 or accompanied by, elevation of this and adjacent portions of North 

 America, interrupted these processes of dissection ; ice, in the form of 

 sheet-glaciers, modified the topography produced in previous epochs, and 

 introduced large amounts of material from the adjacent crystalline area. 

 During the close of the epoch, the time of melting of the glacier, the 

 clay, sand, gravel and boulders which it carried were deposited. The 

 waters collected in great lakes in front of the retreating ice. Around 

 their shores deltas were built, beaches formed, and benches cut ; and a 

 new system of drainage lines was instituted. 



Again, however, changes in relative elevations of different parts, and 

 the withdrawal of the ice, led to the partial dismemberment of the 

 drainage systems, to the definition of the present lake basins, and to the 

 development of new lines of drainage, which are essentially the same 

 to-day, though these and the lake levels are being slowly modified by 

 secular changes of elevation. 



TOPOGRAPHY OF THE PRE-SEDIMENTARY FLOOR. 



Diverse CJiaractcr of the Crystallines. — The crystalline series along 

 the northern boundary of the area comprise rocks in greatest variety, 

 crystalline limestones, micaceous and hornblendic schists, and gneiss, 

 the latter very abundant. Associated with these are plutonic and 

 volcanic rocks, acid, basic, and of intermediate varieties. The whole 

 region has been one of complicated folding and intense metamorphism. 

 The schistose structure of the rocks, throughout the area, is nearly 

 vertical, and has a northeast southwest trend, with local variations 

 from this general direction. 



This great variety of rocks would necessarily offer different resisting 

 powers to erosive agencies, and give rise to very diverse topography. 

 In travelling through the region on foot one is continually ascending or 

 descending. Even then he cannot fail to note the many small tarns, 

 muskegs, and beaver-meadows found so frequently upon the upland 

 areas. 



