270 JDr. J. W. Spencer — Subsidence versus Glacial Dams. 



I have seen. But the drainage was to the north, into the hypo- 

 thetical glacier, which, if it permitted subglacial drainage, could 

 scarcely have formed lakes. 



Under these conditions fairly stated, I think, whether is it easier 

 to accept a great subsidence of the continent to nearly 2700 feet, in 

 Western Pennsylvania, or account for the phenomena by glacial 

 dams, formed on land, vastly lower to the north. 



Indeed, the great deformation of the lake region had scarcely 

 begun, and consequently even the modern highlands, north of the 

 great lakes, were then very much lower than now, when compared 

 with the region to the south. I cannot hesitate in forming a con- 

 clusion that the evidence is in favour of a late continental subsidence, 

 rather than of hypothetical glacial lakes, hundreds of miles long 

 and broad, like nothing ever seen, which could not answer the 

 requirements. 



The difficulty in accepting the subsidence without the occurrence 

 of marine shells has in part been pointed out. But their absence in 

 the lower beaches may be accounted for, in part, by the sheets of 

 water being more or less cut off from the sea, and receiving great 

 quantities of fresh water. This, however, will not explain their 

 absence on the higher beaches. The varying climatic conditions of 

 the water, and the changes of level, destroying the life, and too 

 rapid to allow of remigration, may in part account for the absence of 

 organisms in the shore-lines. 



The record of subsidence deciphered in the high shore-lines of thq 

 lake region is supported by the observations of Dr. G. M. Dawson, 

 Mr. E. G. McConnell, and others, on the mountains rising above the 

 great plains of North- Western Canada, and on the mountains be- 

 tween there and the Pacific Coast. Dr. Dawson ^ finds gravel 

 terraces upon high sides of the Rocky Mountains facing the east, 

 in position showing their origin not to have been river-terraces. 



From extensive observations Dr. Dawson concludes that the 

 Pleistocene submergences amounted to 4000 or 5000 feet in the 

 region of the International Boundary (49th parallel), whilst in 

 Alaska it did not exceed 2500 or 3000 feet. He also hypothecates 

 two episodes of submergence, the latter being less extensive than 

 the former. Further, he regards the elevation and subsidence of the 

 great plains and western mountains as alternating, and that the drift 

 materials of the plains were deposited at sea-level. 



Mr. R. G. McConnell informs us that on Cypress Hills, with an 

 altitude of 4800 feet, the drift does not rise above 4400 feet. A 

 hundred and fifty miles to the north-west the drift is not found 

 above 3400 feet on Hand Hills (Tyrrell). But south of the Cypress 

 Hills, near the 47th parallel, drift occurs up to 4660 feet on Three 

 Buttes (Dawson). From these figures Mr. McConnell shows a 

 differential level of 7-2 feet per mile, the elevation being greater 

 nearer the 49th parallel.'^ 



1 "Later Physiographical Geology of the Eocky Mountain Region in Canada, 

 with Special Eeference to Changes in the Elevation and the History of the Glacial 

 Period," Trans. Eoy. Soc. Canada, 1890. 



2 Geological Survey of Canada, Report for 1885. 



