208 Glaciation of Mountains, Etc. — W. Uphani. 
Canadian watershed between the St. Lawrence and Hudson 
bay 13,000 feet, giving to the ice an average thickness of about 
5,000 feet in the region of the White mountains, 6,500 feet on 
the international boundary, and not less than 12,000 feet on 
the Laurentian highlands.- It still appears to be true that the 
upper limit of the ice-sheet was about 1 ,000 feet below the 
summit of Mt. Washington during the greater part of the Ice 
Age, and that professor Dana's estimates of the thickness of 
the ice farther north are very probable. There seem to be 
good reasons for believing that the land at length sank be- 
neath this heavy burden ; and to that time I would refer the 
complete glacial envelopment of Mt. Washington, as well as 
the transportation of the highest, very scanty drift on Katahdin. 
This depression of the earth's crust led to changes of climate, 
from the rigorous conditions causing glaciation to mild tem- 
peratures by which the ice was finally melted ; but at first the 
subsidence was perhaps attended by an increase in the thick- 
ness of the ice whose surface may have been maintained by 
the snow-fall during a short time geologically speaking, at its 
former altitude, while the area of the White mountains sank 
the 1,000 feet which would envelope the top of Mt. Washing- 
ton in the ice-sheet. The mountain was not thus covered so 
long that the glacial current could sweep away much of the 
abundant frost-riven debris, nor conspicuously emboss any 
projecting knobs of rock, nor bring many bowlders and frag- 
ments of foreign drift. In the two hundred and twenty miles 
from the terminal moraine of Long Island, Martha's Vineyard, 
and Nantucket, north to Mt. Washington, the slope of the ice- 
surface therefore averaged in its maximum, about thirty feet 
per mile compared with the present sea-level and hight of the 
mountain, but was only about twenty-five feet per mile through 
the greater part of the glacial period. It is presumable, how- 
ever, that in a process of subsidence of the land only the thick- 
ness of the ice-sheet, and not the slope of its surface, was in- 
creased when the mountain became wholly ice-covered. 
The bowlders found on Mt. Washington were transported 
by a glacial current moving from northwest to southeast, and 
in the distance of probably fifteen miles from their parent 
ledges to the top of the mountain they were carried upward 
■'American Journal of Science, III, March, 1873, vol. v. pp. 198-211. 
