1868.] on the Mer-de- Glace. 549 



The result completely corroborates the foregoing anticipation. 

 The hinder points are incessantly advancing upon those in front, 

 and that to an extent sufficient to shorten a segment of this glacier, 

 measuring 1000 yards in length, at the rate of 8 inches a day. 

 Were this rate uniform at all seasons, the shortening would amount 

 to 240 feet in a year. When we consider the compactness of this 

 glacier, and the uniformity in the width of the valley which it fills, 

 this result cannot fail to excite surprise ; and the exhibition of force 

 thus rendered manifest, must, in the speaker's opinion, be mainly 

 instrumental in driving the glacier through the jaws of the granite 

 vice at Trelaporte. 



Attention was next drawn to a remarkable system of seams of 

 white ice, which, when observed from a sufficient distance, appears 

 to sweep across the Glacier du Geant, in the direction of the " dirt- 

 bands." These seams are more resistant than the ordinary ice of 

 the glacier, and sometimes protrude above the latter to a height 

 of 3 or 4 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 Talefre, 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 of the ice 

 is traversed by blue veins of a more compact and transparent texture. 

 In the upper portions of the Mer-de-Glace, these veins swept across 

 the glacier in gentle curves, leaning forward, — to which leaning 

 forward. Prof. Forbes gave the name of the " frontal dip." So far 

 as the speaker was informed, a case of " backward dip " had never 

 been described. Yet here, at the base of the ice cascade referred 

 to, he had often noticed the veins exposed upon the walls of a 

 longitudinal crevasse to lean 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 so placed that their ends rest 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 struc- 

 ture before 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. 



Fig. 2 represents a series of these crumples with the veins of 

 white ice i i i at their bases. It was soon observed that the water 

 which trickled down the protuberances, and gushed here and there 



