474 J. W. SPENCER — PLEISTOCENE SUBMERGENCE. 



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 formiug a conclusion that the 

 evidence is in favor of a late continental subsidence rather than in favor of 

 glacial lakes hundreds of miles long and broad, like nothing ever seen, and 

 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, how- 

 ever, will not explain their absence on the higher beaches. The varying 

 climatic conditions of the water and the changes of level destroying the life, 

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

 organisms in the seashore lines. 



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

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

 McConnell and others, on the monuments rising above the great plains of 

 northwestern Canada, and on the mountains between there and the Pacific 

 coast. Dr. Dawson -i^ finds gravel terraces upon the 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 4,000 or 5,000 feet in the region of the inter- 

 national boundary (the 49th parallel), while in Alaska it did not exceed 

 2,500 or 3,000 feet. He also postulates two episodes of submergence, the 

 latter being less extensive than the former. Further, he regards the eleva- 

 tion and subsidence of the great plains and western mountains as alternating, 

 and that the drift material of the plains was deposited at sea-level. 



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

 of 4,800 feet, the drift does not rise above 4,400 feet. One hundred and 

 fifty miles northwestward, the drift is not found above 3,400 feet on Hand 

 hills (Tyrrell); but south of Cypress hills, near the 49th parallel, the drift 

 occurs up to 4,660 feet on Three Buttes (Dawson). From these observations 

 Mr. McConnell shows a differential level of 7.2 feet per mile, the elevation 

 being greater nearer the 49th parallel. 



In the east, the history of the changes has not been fully deciphered. 

 Erratics occur on top of Mount Washington to 6,300 feet, while on Mount 

 Katahdin, in Maine, they occur only to 4,400 feet (Upham). Conforming 

 with Dr. Dawson's views, as applied to the west, we have a greater rise in 

 the White mountains than eastward. The altitude of beach formation on 



*" Later Phy.-iographical Geology of the Rocky Mountain Region in Canada, with Special Refer- 

 ence to Changes in Elevation, and the History of the Glacial Period: " Trans. Roy. Soc. Can., vol. 

 VIII, sec. IV, 1890, pp. 3-74, pis. I-III. 



