314 SURFACE GEOLOGY. 
Nortu AMERICA IN THE IcE PERIOD. 
Candid geologists admit that no part of their knowledge is so obscure 
as that of the cause of glacial cold. Various theories have been sug- 
gested to account for it, but none of them command universal accept- 
ance. Before it can be properly answered, we shall find it necessary 
to consider somewhat the conditions of glacial envelopment, the dimen- 
sions of the areas occupied by the ice, and the directions in which move- 
ment has been effected. In such a study we shall find it necessary to 
look far beyond the confines of New England, for the movements in this 
comparatively limited area are unlike those occupying the greater part 
of the continental ice-sheet. Some of the difficulties we have expe- 
rienced in generalizing our observations result from the smallness of 
our field of study. I will therefore present upon a small map of the 
north-east portion of our continent a delineation of the areas occupied 
by the ice in its period of maximum development, with arrows to indi- 
cate the principal directions of movements. An examination of this 
map, with a brief description of the principal features of glaciation may 
furnish the means for satisfactory generalizations respecting the origin, 
extent, movements, and duration of the ice-sheet. 
The Alps of southern Europe furnish the most accessible example of 
glaciers in action, with indications of greater extent in the ice-period. 
The higher portions of the range are occupied by immense fields of 
snow, which are the source of the numerous glaciers pushing out from it 
on every side, both towards Switzerland and France on the north, and 
towards Italy upon the south. These glaciers may be said to radiate 
from a central line of dispersion. In the Alpine district the accessible 
glaciers behave like streams of water, occupying only the bottoms of the 
valleys, and descending to the lower levels apparently in obedience to 
the laws of gravity. A study of the former extent of these glaciers indi- 
cates their former extension across the valley of Switzerland to the Jura 
mountains. The Rhone glacier moved over the great valley to the Jura 
mountains, occupying an area 50 miles wide, 150 long, and 2,000 feet 
deep. Lateral, medial, terminal, and ground moraines occur in connec- 
tion with all these glaciers, Large boulders have been transported from 
Mt. Blanc to the Jura. 
