ARGUMENT FROM RATES OF PROGRESSION. 
197 
would divide the glacier even where most level into trapezia, and no transverse 
crevasse could be straight-edged but must be jagged like a saw, or cut en Echelon. 
Such a phenomenon never occurs unless where a glacier is moving torrentiaUy , or with 
great disturbance and down a steep. There such longitudinal fissures may occasion- 
ally be seen, but they form the exception and not the rule. It has been demonstrated 
by an elaborate proof in § 5, that the only trace of longitudinal discontinuity in the 
normal condition of the glacier is to be found in the veined structure, which, being 
caused by a partial discontinuity at a vast number of points, admits of an insensible 
deformation of the glacial mass without sudden or complete rents, or slips, or the 
formation of zigzag crevasses. 
The existence of the great transverse crevasses, which, even in glaciers not moving 
torrentiaUy, divide the surface of a glacier by rents perhaps 2000 feet long*, have 
been thought by some to be comparable to beams of an elastic material, supported 
at the two ends, and bending under their own weight forward, in the middle. Were 
this the case, it would scarcely modify the plastic theory as I have propounded it ; 
because in order that such a bar of ice should conform to the known movements of 
the glacier, opposite the Montanvert for instance, the centre must continually gain 
upon the sides at the rate of 150 feet per annum at least, consequently the limit of 
cohesion of an elastic solid would soon be overpassed, and plasticity in the material 
sufficient to explain the whole motion would inevitably be admitted at last. Inde- 
pendently of this, it is evident, that were such a flexure essential to the motion, the 
lines of crevasses would be convex in the direction in which the glacier is moving 
instead of towards its origin. 
Argument from the Equable Progression of Glaciers. — The equability of the motions 
of the various parts of a glacier, united as I have shown them to be by intricate rela- 
tions-j-, must, I think, appear conclusive to every one capable of forming a just opi- 
nion on the subject, that the relative movements of the various parts of the glacier 
are due to the action of forces at small distances and to the antagonism of molecular 
cohesions and molecular strains, and not to the casual jumbling of a quantity of rude 
fragments. To myself, I confess that this now appears the strongest argument of all 
for considering the glacier as a united mass like a river, in which there is a nice equi- 
librium between the force of gravitation, acting by hydrostatic pressure, and the mole- 
cular resistances of the semi-solid ; the degree of regularity of the law which con- 
nects the partial movements is wonderful, and I maintain that it is inexplicable except 
upon the viscous theory. Thus (1) the glacier moves continually, summer and winter, 
day and night, and never by fits or starts ; for if it does — if gravitation overcomes 
mere friction, it occasions a shock or avalanche ; (2) its mean annual motion is nearly 
alike from year to year ; (3) the relative velocities of points widely distributed over 
the glacier (but exposed to similar influences of climate), change simultaneously in 
the same directions, often in the same proportions; thus the variation of velocity 
* Travels, p. 171, 2nd ed. + See § 5 of this paper, pages 167 and 168. 
