18 GLACIAL AND SURFACE GEOLOGY OF THE PACIFIC COAST. 



scale, while the latter are more local in character. Hence, when such sub- 

 raarine deposits are raised above the surface, and in that elevation have been 

 not at all or only very slightly disturbed, the drainage of the uplifted region 

 will be eflfected simply by rivers, there Ijeing few depressions to be filled 

 with the excess of precipitation and thus to become lakes. Highly meta- 

 morphic rocks, on the other hand, are not only by their nature impervious 

 to water, but they must have been left, after passing through the various 

 stages of chemical change and orogi-aphic displacement, with an uneven siu'- 

 face, suited to give rise to collections of standing water. This result, under 

 suitable climatological conditions, cannot fail to take place, unless the irregu- 

 lar surface is afterwards subjected to some erosive agency which smooths 

 it over and reduces it to one uniform level. Erosion of this kind can, 

 however, but rarely take place, for the ordinary denuding agents are quite 

 as likely to deepen the previously existing depressions as to obliterate them. 

 A level surface may indeed be produced in such cases by the entire surface 

 becoming uniformly covered with detrital material ; but this can only ])e 

 effected by a submergence of the region beneath the sea, during a long- 

 period and under favorable conditions. 



Keeping the above considerations in view, it will be evident why regions 

 like Canada and Finland are covered with a network of small lakes. The 

 countries in question are underlain by impermeable rocks, which have dur- 

 ing an immense period been subjected to subaerial erosion. The overlying 

 detritus is usually quite thin, and only here and there accumulated in heavy 

 masses. It consists largely of gravel, but has enough clayey material con- 

 nected with it to ffivor the formation of lakes and ponds. Water has a 

 tendency to accumulate in the depressions of the solid rock, and this ten- 

 dency is farther aided by the peculiar manner in which the eroded materials 

 have been scattered over the surface. The impervious character of the 

 imderlying rock, the thinness and irregular distributiou of the overlying 

 detritus, and the general uniformity of level of the region, — these are the 

 agencies which have given rise to the multiplicity of lakes in the countries 

 in question. 



Thus we see why, as soon as we leave the region of crystalline rocks, 

 either in Northwestern Russia or Northeastern America, we leave the lakes 

 behind us, as already indicated. That glacial erosion is not the cause of the 

 sudden change in this respect, is shown clearly enough by the fact that wo 

 pass at once, in the midst of an area formerly covered with ice, as would be 



