42 
CAUSES OF GLACIER MOTION. 
continuous descending streams, that as effectually drain and 
keep down the level of the great snowfields, as rivers drain 
and keep down the level of lakes. In consequence, however, 
of the glaciers being frozen water, their motion is very 
different to that of liquid water, and becomes complicated by 
several interfering causes. 
The snow suffers compression and consolidation from the 
weight of the accumulated mass, and this pressure becomes 
so intense under the enormous masses that the snow crystals 
are ultimately welded together into a solid transparent block 
of ice in the same way as two blocks of ice can be united by 
sufficient pressure, the two blocks being united into a single 
undivided block as completely as two pieces of iron are 
welded into one when pressed together at the right 
temperature. The cementing together of the snow is further 
aided by the trickling down of the surface water from the 
partial melting of the snow that is caused by the heat of the 
sun’s direct rays in the middle of each day. 
The glaciers are irregular streams of this ice, extending to 
half a'mile'in width, 300ft. in thickness, and six miles in 
length, in Switzerland; but they are considerably larger in 
other countries, and especially in Greenland, where the largest 
known glaciers exist, one of which is as much as three miles 
in width and thirteen miles in length. These, however, 
appear quite insignificant when compared with the gigantic 
glaciers, of which there are distinct traces, from the “ Great 
Ice Age,” when the Rhone Glacier in Switzerland, for instance, 
must have be!en no less than 120 miles in length and 2,000ft. 
in thickness. 
In the motion of glaciers, gravity, instead of being the 
sole moving cause as in the case of rivers, is only sufficient to 
account for a small portion of the work that is done. The 
velocity of motion of water in rivers bears direct relation to 
their rate of fall or inclination, but the velocity of motion of 
ice in glaciers bears no relation whatever to the inclination of 
the glacier. The most rapid moving glacier—the one in 
Greenland already referred to—is very flat, and has much 
less inclination than the slow moving glaciers. The Mer de 
Glace in Switzerland, with a slope of one in twelve, does not 
move faster than 3ft. per day ; but the Greenland glacier, 
which has a slope of only about one in one hundred and 
twenty, has a motion of as much as C5ft. per day, or twenty- 
two times the velocity, with only one-tentli of the inclination. 
Some other cause than gravity has, therefore, to be looked 
for, and various ingenious theories have been proposed to 
explain the motion of glaciers; but these have failed to satisfy 
