The head of Mont Blanc is supported on this side by two buttresses, between which vast glaciers descend. Of these the most southern(134) takes its rise at the foot of the precipices which fall steeply down from the Calotte,(135) and its stream, as it joins that of the Miage, is cut in two by an enormous _rognon_ of rock. Next, to the left, comes the largest of the buttresses of which I have spoken, almost forming an aiguille in itself. The next glacier(136) descends from a large basin which receives the snows of the summit-ridge between the Bosse and the Dome, and it is divided from the third and last glacier(137) by another buttress, which joins the summit-ridge at a point between the Dôme and the Aig. de Bionnassay.] The great buttresses betwixt these magnificent ice-streams have supplied a large portion of the enormous masses of débris which are disposed in ridges round about, and are strewn over, the termination of the Glacier de Miage in the Val Véni. These moraines(138) used to be classed amongst the wonders of the world. They are very large for a glacier of the size of the Miage. The dimensions of moraines are not ruled by those of glaciers. Many small glaciers have large moraines,(139) and many large ones have small moraines. The size of the moraines of any glacier depends mainly upon the area of rock surface that is exposed to atmospheric influences within the basin drained by the glacier; upon the nature of such rock,—whether it is friable or resistant; and upon the dip of strata. Moraines most likely will be small if little rock surface is exposed; but when large ones are seen, then, in all probability, large areas of rock, uncovered by snow or ice, will be found in immediate contiguity to the glacier. The Miage glacier has large ones, because it receives detritus from many great cliffs and ridges. But if this glacier, instead of lying, as it does, at the bottom of a trough, were to fill that trough, if it were to completely envelope the Aiguille de Trélatête, and the other mountains which border it, and were to descend from Mont Blanc unbroken by rock or ridge, it would be as destitute of morainic matter as the great _Mer de Glace_ of Greenland. For if a country or district is _completely_ covered up by glacier, the moraines may be of the very smallest dimensions.(140) The contributions that are supplied to moraines by glaciers themselves, from the abrasion of the rocks over which their ice passes, are minute compared with the accumulations which are furnished from other sources. These great rubbish-heaps are formed, one may say almost entirely, from débris which falls, or is washed down the flanks of mountains, or from cliffs bordering glaciers; and are composed, to a very limited extent only, of matter that is ground, rasped, or filed off by the friction of the ice.