THE GLACIER SLIDES.
Another early surmise was that of Altmann and Grüner (1760), both of whom conjectured that the glacier slid along its bed. This theory received distinct expression from De Saussure in 1799; and has since been associated with the name of that great alpine traveller, being usually called the 'Theory of Saussure,' and sometimes the 'Sliding Theory.' It is briefly stated in these words:—
"Almost every glacier reposes upon an inclined bed, and those of any considerable size have beneath them, even in winter, currents of water which flow between the ice and the bed which supports it. It may therefore be understood that these frozen masses, drawn down the slope on which they repose, disengaged by the water from all adhesion to the bottom, sometimes even raised by this water, must glide by little and little, and descend, following the inclinations of the valleys, or of the slopes which they cover. It is this slow but continual sliding of the ice on its inclined base which carries it into the lower valleys."[A]
STRAINED INTERPRETATION.
De Saussure devoted but little time to the subject of glacier-motion; and the absence of completeness in the statement of his views, arising no doubt from this cause, has given subsequent writers occasion to affix what I cannot help thinking a strained interpretation to the sliding theory. It is alleged that he regarded a glacier as a perfectly rigid body; that he considered it to be "a mass of ice of small depth, and considerable but uniform breadth, sliding down a uniform valley, or pouring from a narrow valley into a wider one."[B] The introduction "of the smallest flexibility or plasticity" is moreover emphatically denied to him.[C]
It is by no means probable that the great author of the 'Voyages' would have subscribed to this "rigid" annotation. His theory, be it remembered, is to some extent true: the glacier moves over its bed in the manner supposed, and the rocks of Britain bear to this day the traces of these mighty sliders. De Saussure probably contented himself with a general statement of what he believed to be the substantial cause of the motion. He visited the Jardin, and saw the tributaries of the Mer de Glace turning round corners, welding themselves together, and afterwards moving through a sinuous trunk-valley; and it is scarcely credible that in the presence of such facts he would have denied all flexibility to the glacier.
The statement that he regarded a glacier to be a mass of ice of uniform width, is moreover plainly inconsistent with the following description of the glacier of Mont Dolent: "Its most elevated plateau is a great circus, surrounded by high cliffs of granite, of pyramidal forms; thence the glacier descends through a gorge, in which it is narrowed; but after having passed the gorge, it enlarges again, spreading out like a fan. Thus it has on the whole the form of a sheaf tied in the middle and dilated at its two extremities."[D]
GLACIER OF MONT DOLENT.
Curiously enough this very glacier, and these very words, are selected by M. Rendu as illustrative of the plasticity of glaciers. "Nothing," he says, "shows better the extent to which a glacier moulds itself to its locality than the form of the glacier of Mont Dolent in the Valley of Ferret;" and he adds, in connexion with the same passage, these remarkable words:—"There is a multitude of facts which would seem to necessitate the belief that the substance of glaciers enjoys a kind of ductility which permits it to mould itself to the locality which it occupies, to grow thin, to swell, and to narrow itself like a soft paste."[E]