124. I would now ask you to consider for a moment the facts which such an excursion places in our possession. The snow through which we have in idea trudged is the snow of last winter and spring. Had we placed last August a proper mark upon the surface of the snow, we should find it this August at a certain depth beneath the surface. A good deal has been melted by the summer sun, but a good deal of it remains, and it will continue until the snows of the coming winter fall and cover it. This again will be in part preserved till next August, a good deal of it remaining until it is covered by the snow of the subsequent winter. We thus arrive at the certain conclusion that on the plateau of the Col du Géant the quantity of snow that falls annually exceeds the quantity melted.
125. Had we come in the month of April or May, we should have found the glacier below the ice-fall also covered with snow, which is now entirely cleared away by the heat of summer. Nay, more, the ice there is obviously melting, forming running brooks which cut channels in the ice, and expand here and there into small blue-green lakes. Hence you conclude with certainty that below the ice-fall the quantity of frozen material falling upon the glacier is less than the quantity melted.
126. And this forces upon us another conclusion: between the glacier below the ice-fall and the plateau above it there must exist a line where the quantity of snow which falls is exactly equal to the quantity annually melted. This is the snow-line. On some glaciers it is quite distinct, and it would be distinct here were the ice less broken and confused than it actually is.
127. The French term névé is applied to the glacial region above the snow-line, while the word glacier is restricted to the ice below it. Thus the snows of the Col du Géant constitute the névé of the Glacier du Géant, and in part, the névé of the Mer de Glace.
128. But if every year thus leaves a residue of snow upon the plateau of the Col du Géant, it necessarily follows that the plateau must get annually higher, provided the snow remain upon it. Equally certain is the conclusion that the whole length of the glacier below the cascade must sink gradually lower, if the waste of annual melting be not made good. Supposing two feet of snow a year to remain upon the Col, this would raise it to a height far surpassing that of Mont Blanc in five thousand years. Such accumulation must take place if the snow remain upon the Col; but the accumulation does not take place, hence the snow does not remain on the Col. The question then is, whither does it go?
SKETCH-PLAN, SHOWING THE MORAINES, a, b, c, d, e, OF THE MER DE GLACE.
[§ 16.] Branches and Medial Moraines of the Mer de Glace from the Cleft Station.