1872.] | 91 [Perry. 
of a mass from five thousand to ten thousand feet in thickness, mo- 
tion toward the south would inevitably result, even on a. horizontal 
plane, and much more, if there were a southward inclination of the 
country. A southerly motion being thus almost a matter of neces- 
sity, its probable rate becomes a question of interest, and deserves a 
passing glance. In looking at this point, one ought not to forget the 
marked difference in the conditions under which the great ice-sheet 
apparently existed, and those connected with the short, isolated 
glaciers of modern times. We should remember that we have no 
positive evidence that the face of the country was any more broken 
or mountainous, during the glacial period, than it is to-day; that the 
mass of ice probably rested on a surface not so far removed from a 
plain as some suppose; that in addition to the comparative absence 
of slope in the region, it had occasional mountain ridges, many hills, 
and other kindred obstructions to surmount; and that, consequently, 
its motion must have been very slow, as compared with that of any 
of the existing glaciers of the Alps. We are also to bear in mind 
that there must have been considerable variation in this movement, 
in different localities, determined by the existing exigencies, as the 
equality or inequality of the surface, the degrees of its inclination, 
the thickness of the ice-mass, and the like. 
Now, while accurate measurements have been made! of the mo- 
tion of glaciers of different dimensions, slope, thickness, age, and 
the like, in the steep, narrow valleys of the Alps, we can hardly ap- 
ply these, without modification, to the broad and comparatively level 
ice-sheet, as it probably existed in this country. Indeed, an atten- 
tive examination of a great number of facts has forced me to believe 
that the movement in New England was extremely slow. Professor 
Dana suggests that the glacial sheet had a motion ‘‘ not exceeding a 
mile in a century, which is equivalent to about a foot a week.” ? 
This estimate, as should be remembered, was made on the supposi- 
tion that the northern part of the continent was elevated during the 
ice period, and that thus the slope was then much greater than it is. 
now. Various considerations indicate, that the rate of movement, : 
in most localities, was by no means so great as the estimate implies; 
while in some, it may have been greater. ‘The conviction wakened, 
as the result of considerable observations, is that the average rate 
1 See “Systeme Glaciaire,”’ by Prof. Agassiz. 
2 Paper cited, p. 50. 
