630 ALFRED IV. G. WILSON 



to the peneplain condition by the processes of subaerial degrada- 

 tion. 



We have already seen that the Laurentian peneplain exhibits 

 to a most marked degree the even sky-line and low surface 

 gradients. We find, however, that it is almost devoid of mantle 

 rock i?t situ. That which occurs, with the exception of an 

 exceedingly small amount, almost too small to notice, was brought 

 to its present location by glacial or aqueous processes of trans- 

 portation. The drainage over most parts of the area is not well 

 established ; rapids and waterfalls are the most characteristic 

 features of all the streams. The entire surface of the country is 

 dotted with lakes, which are practically numberless ; in places 

 their area approaches 25 per cent, of the whole. And finally, 

 parts of the peneplain are not now in accordant position with 

 respect to sea-level. Thus, although the complicated and contorted 

 rocks of the region are everywhere truncated by a surface which 

 presents a remarkably even sky-line, in almost every other respect 

 the features of the region are different from those characteristic 

 of a peneplain. Since the production of the peneplain surface 

 which traverses the rocks of the region, the surface of the plain 

 has undergone profound modification, leading to the production 

 of the present features. 



In the descriptions which follow, the general nature of these 

 departures from the normal type of peneplain will be described 

 more in detail, and at the end of the paper a brief reference will 

 be made to the processes which may have brought about these 

 modifications. In general it may be said that the present attitude 

 of different parts of the plain with respect to sea-level is due to 

 a differential uplift, which, in parts of the area at least, is still 

 going on. The later dissection of the plain, the carving of deep, 

 broad valleys or of narrow canyons, seems to be a function of 

 this uplift, though many of the first class of depressions here 

 mentioned may antedate the peneplain. The absence of mantle 

 rock i?i situ, the occurrence of the numerous lake basins, and the 

 inmaturity of the drainage systems are usually attributed to the 

 erosive action of glacial ice. 



3. Detailed descriptions of characteristic portio7is of the plaiii. — 



