But how is the backward dip of the blue veins to be accounted for? Doubtless in the following way: At the base of the cascade the glacier is forcibly compressed by the thrust of the mass behind it; besides this, it changes its inclination suddenly and considerably; it is bent upwards, and the consequence of this bending is a system of wrinkles, such as those represented in the next figure. The interior of a bent umbrella-handle sometimes presents wrinkles which are the representatives, in little, of the protuberances upon the glacier. The coat-sleeve is an equally instructive illustration: when the arm is bent at the elbow the sleeve wrinkles, and as the places where these wrinkles occur in the cloth are determined, to some extent, by the previous creasing, so also the places where the wrinkles are formed upon the glacier are determined by the previous scarring of the ice during its descent down the cascade. The manner in which these crumples tend to _scale off_ speaks strongly in favour of the explanation given. The following figure represents a type of numerous instances of scaling off. By means of a hydraulic press it is easy to produce a perfectly similar scaling in small masses of ice. One consequence of this crumpling of the glacier would be the backward and forward inclination of the veins as actually observed. The same appearance was noticed on the wrinkles of the Glacier du Géant. It was also proved, by measurements, that these wrinkles _shorten_ as they descend.
In virtue of what quality, then, can ice be bent and squeezed, and have its form changed in the manner indicated in the foregoing observations? The only theory worthy of serious consideration at the present day is the celebrated Viscous Theory of glacial motion. Numerous appearances, as we have seen, favour the idea that ice is a viscous or 'semi-fluid' substance, and that it flows as such in the glaciers of the Alps. The aspect of many glaciers, as a whole--their power of closing up crevasses, and of reconstructing themselves after having been precipitated down glacial gorges--the obvious bendings and contortions of various portions of the ice, are all in harmony with the notion. The laminar structure of the glacier has also been regarded by eminent authorities as a crucial test in favour of the viscous theory, and affirmed to be impossible of explanation on any other hypothesis.