It is difficult to convey by writing the force of the evidence which the actual observation of this natural experiment places before the mind. The ice at the base of the fall, retarded by the gentler inclination of the valley, has to bear the thrust of the descending mass, the sudden change of inclination producing powerful longitudinal compression. The protuberances are squeezed more closely together, the hollows between them appear to wrinkle up in submission to the pressure—in short, the entire aspect of the glacier suggests the powerful operations of the latter force. At the place where it is exerted the veined structure makes its appearance; and being once formed, it moves downwards, and gives a character to other portions of the glacier which had no share in its formation.
BASE OF CASCADE A "STRUCTURE-MILL."
An illustration almost as good, and equally accessible, is furnished by the Glacier of the Rhone. I have examined the grand cascade of this glacier from both sides; and an ordinary mountaineer will find little difficulty in reaching a point from which the fall and the terminal portion of the glacier are both distinctly visible. Here also he will find the cliffy ridges separated from each other by transverse chasms, becoming more and more subdued at the bottom of the fall, and disappearing entirely lower down the glacier. As in the case of the Grindelwald Glacier the squeezing of the protuberances and of the spaces between them, is quite apparent, and where this squeezing commences the transverse structure makes its appearance. All the ice that forms the lower portion of this glacier has to pass through the structure-mill at the bottom of the fall, and the consequence is that it is all laminated.
STRUCTURE OF RHONE GLACIER.
TRANSVERSE STRUCTURE.
This case of structural development will be better appreciated on reference to [Figs. 41] and [42], the former of which is a plan, and the latter a section, of a part of the ice-fall and of the glacier below it; a b e f is the gorge of the fall, f b being the base. The transverse cliffy ice-ridges are shown crossing the cascade, being subdued at the base to protuberances which gradually disappear as they advance downwards. The structure sweeps over the glacier in the direction of the fine curved lines; and I have also endeavoured to show the direction of the radial crevasses, which, in the centre at least, are at right angles to the veins. To the manifestation of structure here considered I have, for the sake of convenient reference, applied the term transverse structure.
A third exhibition of the structure is now to be noticed. We sometimes find it in the middle of a glacier and running parallel to its length. On the centre of the ice-fall of the Talèfre, for example, we have a structure of this kind which preserves itself parallel to the axis of the fall from top to bottom. But we discover its origin higher up. The structure here has been produced at the extremity of the Jardin, where the divided ice meets, and not only brings into partial parallelism the veins previously existing along the sides of the Jardin, but develops them still further by the mutual pressure of the portions of newly welded ice. Where two tributary glaciers unite, this is perhaps without exception the case. Underneath the moraine formed by the junction of the Talèfre and Léchaud the structure is finely developed, and the veins run in the direction of the moraine. The same is true of the ice under the moraine formed by the junction of the Léchaud and Géant. These afterwards form the great medial moraines of the Mer de Glace, and hence the structure of the trunk-stream underneath these moraines is parallel to the direction of the glacier. This is also true of the system of moraines formed by the glaciers of Monte Rosa. It is true in an especial manner of the lower glacier of the Aar, whose medial moraine perhaps attains grander proportions than any other in the Alps, and underneath which the structure is finely developed.
LONGITUDINAL STRUCTURE.