230 The American Geologist. October, 1902, 
in the central portions of the surface, since the friction between 
the mountain sides and the ice particles is greater than that 
between the ice particles themselves. This active mechanical 
stress results in a differential motion of the ice mass which is 
of the nature of both plane and torsional shear. The stress 
is great enouj^^h to overcome the strength of the solid and 
the ice yields by snapping asunder, thus forming a miniature 
crevasse. The mechanical stress may also be aided mere or 
less in the formation of cracks, by the factor of contraction 
wliich tends to take place in the ice as the temperature de- 
creases. The surface layers are especially susceptible to this 
influence, since the temperature of the air above the ice is like- 
ly to be at times far below the freezing point. Furthermore 
cracks once started would not be nearly filled at once with 
water from below, as is freciuently the case with lake ice, * 
Init would, so long as mechanical stress and contraction con- 
tinue open wider and deeper, finally resulting in vast impas- 
sable chasms. 
If the ice travels over a very uneven bed, as is often the 
case with Alpine glaciers, some portions become engulfed, 
while the upper parts slide over them, and here the forma- 
tion of immense crevasses is greatly augmented. Evidence 
is plentiful to show, however, that the upper portion of nearly 
all glaciers is traversed by fissures or crevasses, regardless 
of angle of slope, or volume of the ice mass. These fissures 
beginning as very small cracks will increase in size until a 
more equable slope is reached, or, a rise in temperature occurs, 
when there is at once a tendency for the crevasses to close, 
and the ice regains its former solid condition by regelation. 
Again and again are repeated the before mentioned processes 
of rupture by mechanical strain and contraction, with sub- 
sequent repair by pressure and regelation and thus the glacier 
ice in the zone of fracture moves onward down the slope. 
The movement of ice in this zone would be expected to 
comply with observed conditions, viz. : a more rapid rate in 
summer than in winter, and a greater movement by day than 
by night. During the time when the temperature of the air 
in contact with the ice is falling, the resistance to the onward 
moving force of gravitv is increased, and contraction of the 
*"Ice Hampai-ts." E. - R. Buckley and C. R. Van Hise. Trans, of the 
Wis. Academy of Sciences, Arts, and Letters, Vol. 13. 
