THE GREAT ICE AGE 369 



there were marine conditions in the lower and middle St. 

 Lawrence and in the Ottawa valley, and swamps and lakes on 

 the upper Ottawa and the western end of Lake Ontario. It is 

 quite probable, nay, certain, that during this interglacial period 

 re-elevation had set in, since the upper Leda clay and the 

 Saxicava sand indicate shallowing water, and during this re- 

 elevation the plant-covered surface would extend to lower levels. 



This, however, must have been followed by a second subsi- 

 dence, since the water-worn gravels and loose, far-travelled 

 boulders of the later drift rose to heights never reached by the 

 till or the Leda clay, and attained to the tops of the highest 

 hills of the St. Lawrence valley, 1,200 feet in height, and else- 

 where to still greater elevations. This second boulder drift 

 must have been wholly marine, and probably not of long 

 duration. It shows no evidence of colder climate than that 

 now prevalent, nor of extensive glaciers on the mountains ; 

 and it was followed by a paroxysmal elevation in successive 

 stages till the land attained even more than its present height, 

 as subsidence is known to have been proceeding in modern 

 times. 



I am quite aware that the above sequence and the causes 

 assumed are somewhat different from those held by many 

 geologists with reference to regions south of Canada ; but must 

 hold that they are the only rational conclusions which can be 

 propounded with reference to the facts observed from the 

 parallel of 45° to the Arctic Ocean. 



My own observations have been chiefly in the eastern part 

 of North America. My son. Dr. G. M. Dawson, has much 

 more ably and thoroughly explored those of the west ; and 

 after describing the immense Cordilleran ice mass which ex- 

 tended for a length of 1,200 miles along the mountains of 

 British Columbia and discharged large glaciers to the north, as 

 well as to the west and south, and stating his reasons for 

 believing in that differential elevation and depression which 



