THE INFLUENCE OF DEBRIS ON THE FLOW OF 
(GIL AMCIU SIRS. 
THE behavior of ice under various conditions is frequently 
illustrated by experiments with pitch or other similar viscous 
fluids or plastic solids. If sand or other similar substance is 
distributed through pitch its plasticity will be reduced. When 
sufficient sand is added, the pitch will no longer flow under con- 
ditions that will cause clear pitch to readily change its shape. 
The reason for this is manifest; by mingling a rigid substance 
with one that is plastic the rigidity of the latter will be increased. 
As the percentage of rigid material increases the compound sub- 
stance acquires more and more of its characteristics. Let us 
apply this principle to glaciers. 
Ice under pressure behaves as a plastic solid. When a mass 
of ice, unsupported at the sides, is sufficiently large, it will 
change its shape of flow under the influence of its own weight. 
Although objections have been raised to each of these propo- 
sitions, I shall for the present consider them demonstrated. 
The study of glaciers, especially of the Alpine type, has 
shown that they flow after the manner of plastic solids. That is, 
there is a differential motion of molecules, or of particles, 
throughout the mass. In most instances, it seems safe to 
assume no two points, in a cross section of a glacier, will move 
at the same rate for any considerable time. 
If we conceive of a glacier composed of clear ice moving at 
a given rate, and introduce débris — earth, sand, stones, bowlders, 
etc.—into it, without altering other conditions, the effect will be 
to decrease the rate of flow, since rigid substances are added to 
one that is plastic. If we gradually increase the percentage of 
débris, the ice becomes less and less plastic and finally acquires 
such rigidity that under the conditions normally influencing the 
movements of glaciers, it will cease to flow. If the débris instead 
of being uniformly commingled with the ice, is introduced irreg- 
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