The Flotr of Glaciers. — Upkam. 25 
When a plastic or viscous substance undergoes change of form with- 
out change of bulk, the distortion in its simplest form may be regarded 
as clue to the formation of great numbers of parallel shear planes. In 
such a case every molecule of each plane must change its position with 
respect to every other layer of molecules. On the other hand, if the 
substance be built up of a number of rigid grains of all shapes and sizes, 
closely fitting and adhering to each other, the nature of the change ne- 
cessary to give rise to distortion is much less simple. In such a case not 
only do we require shearing between the interfaces of the particles but 
also a change of shape of the particles themselves. And this must go 
on in ice without producing more than local ruptures, for its tenacity 
and shearability are sufficiently high to resist general fracture. To ac- 
c< >u nt for Glacier-Motion, therefore, we have to show that the glacier 
grains can not only increase in size but also change their shapes under 
the smallest stresses, and also that they can. under similar conditions, 
slide over each other without actual fractures resulting. 
We will first consider the question of change of shape and size. Fig. 
8 shows an ideal case of a number of particles lying between two paral- 
lel planes, the upper of which is moving more rapidly than the lower 
one. The small arrows near to or crossing the interfaces indicate the 
direction in which shear must take place, and also show those surfaces 
which, being pressed together, must be wasting, and those surfa- 
ces which, being in tension, must be growing. Although we shall deal 
with the case as though each crystal had rectilinear motion only, it 
must lie remembered that they will have a tendency to roll over each 
other as well. This, however, rather reduces than adds to thedifficulty 
of the problem, as does also the viscosity of ice-grains along planes at 
light angles to the optic axis. We have seen that a very large number 
of the molecules at the interfaces of the crystals are free: that is, they 
sometimes form portions of one crystalline structure, and sometimes of 
another. Probably within a few minutes all thesurface molecules have 
been free, and have, therefore, been at liberty to assume positions more 
in accordance with the conditions of stress and strain in the mass. For 
instance, the conditions of stress will be different for each of the faces 
separating the three crystals, 2. 4, and 5 of Figure K. One is in com 
pression, the other in tension, and the third in shear, and consequently 
the structure between 4 and '2 is more open (being in tension i than that 
between 4 and .1. the adjacent faces of which are in compression. Un- 
der such circumstances, it is reasonable to suppose that there would be 
a migration of molecules from the opposed faces of 4 and 5 to the op 
posed faces of 4 and 2. An exceedingly slow change of this kind would 
lie sufficient for our purpose. There now remain the faces, such as 
those between '1 and 5, to consider. In these the case is one of simple 
shear. 
This explanation, which may perhaps properly lie called a 
Granulation Theory'of glacier motion, appears more nearly 
allied to the view of Forbes than to that of Tvndall. Yet. if 
