48 THE ICE AGE IN CANADA. 



plains and elevation of mountains. There is reason to 

 believe that such alternations were not infrequent in the 

 Pleistocene, and that their occurrence will explain many 

 of the complexities of these deposits. 



If we adopt for the more general deposits the hypothesis 

 of floating ice, we must be prepared to consider in con- 

 nection with tliis subject a subsidence so great as to place 

 at one period all but the highest parts of the Laurentides 

 and Appalachians under water. In this case a vast 

 volume of arctic ice and water would pour over the 

 country of the great lakes to the S.W., while any obstruc- 

 tion occurring to the south would throw lateral currents 

 over the Appalachians to the eastward. 



It is evident from the descriptions of Smith, Geikie, 

 Jameson, Crosskey, and others, that the boulder-clay 

 of Scotland and Scandinavia corresponds precisely in 

 character with that of Canada, and there, as in America, 

 the theory of a continental glacier has been resorted to 

 for its explanation. The objections to this hypothesis are 

 very ably stated by Mr. Milne Home in a paper on the 

 " Boulder-clay of Europe," in the Transactions of the 

 Royal Society of Edinburgh, 1869. 



To this period and these causes must also be assigned 

 the excavation of the basins of the great American lakes. 

 These have been cut out of the softer members of the 

 Silurian and Devonian Formations ; but the mode of this 

 excavation has been regarded as very mysterious ; and, 

 like other mysteries, has been referred to glaciers. Its 

 real cause was obviously river and atmospheric erosion in 

 the Pliocene period, supplemented by the flowing of cold 

 ocean currents over the American land during its sub- 

 mergence.* The lake-basins are thus of the same nature 



* See Chapter III. 



