When the Glacier du Géant is observed from a sufficient distance, a remarkable system of seams of white ice appears to sweep across it, in the direction of the ‘dirt-bands.’ These seams are more resistant than the ordinary ice of the glacier, and sometimes protrude above the surface to a height of three or four feet. Their origin was for some time a difficulty, and it was at the base of the ice-cascade which descends from the basin of the Talèfre that the key to their solution first presented itself. It was well known that the ice of a glacier is not of homogeneous structure, but that the general more or less milky mass is traversed by blue veins of a more compact and transparent texture. In the upper portions of the Mer de Glace these veins sweep across the glacier in gentle curves, leaning forward—to which leaning forward Prof. Forbes gave the name of the ‘frontal dip.’ A case of ‘backward dip’ has never been described. But at the base of the ice-cascade referred to I had often noticed the veins exposed upon the walls of a longitudinal crevasse leaning backwards and forwards on both sides of a vertical line, like the joints of stones used to turn an arch.
This fact was found to connect itself in the following way with the general state of the glacier. At the base of the ice-fall a succession of protuberances, with steep frontal slopes, followed each other, and were intersected by crevasses. Let the hand be placed flat upon the table, with the palm downwards; let the fingers be bent so as to render the space between the joints nearest the nails and the ends of the fingers nearly vertical. Let the second hand be now placed upon the back of the first, with its fingers bent as in the former case, and their ends resting upon the roots of the first fingers. The crumpling of the hands fairly represents the crumpling of the ice, and the spaces between the fingers represent the crevasses by which the protuberances are intersected. On the walls of these crevasses the change of dip of the veined structure above referred to was always observed, and at the base of each protuberance a vein of white ice was found firmly wedged into the mass of the glacier.
The next figure represents a series of these crumples with the veins of white ice i i i at their bases.
Fig 2.
It was soon observed that the water which trickled down the protuberances, and gushed here and there from glacier orifices, collected at the bases of the crumples, and formed streams which cut for themselves deep channels in the ice. These streams seemed to be the exact matrices or moulds of the veins of white ice, and the latter were finally traced to the gorging up of the channels of glacial rivulets by winter snow. The same explanation applies to the system of bands upon the Glacier du Géant. I was enabled to trace the little arms of white ice which once were the tributaries of the streams, to see a trunk vein of the ice dividing into branches, and uniting again so as to enclose glacial islands. I finally traced them to the region of their formation, and by sketches of existing streams taken near the base of the séracs, and of bands of white ice taken lower down, a resemblance so striking was exhibited as to leave no doubt of their relationship. On the walls of some deep crevasses, moreover, which intersected the white ice-seams, I found that the latter penetrated the glacier only to a limited depth, having the appearance of a kind of glacial ‘trap’ intruded from above.
FIG. 3.