Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/464

 therefore the valleys themselves, were mainly excavated before the glacial epoch, and that the effect produced upon them by the glaciers was, comparatively speaking, superficial*.

It remains therefore to ask, what agent, or agents, have produced these remarkable natural amphitheatres, and those minor and less perfect copies so common in the limestone districts, though not wholly confined to them ?

In the descriptions of these cirques, attention has been called to the numerous streamlets seaming their cliffs, which were most of all conspicuous in the two under the Rothstock peaks.

Running water, in falling over a cliff, notches the edge, grooves the face, and undermines the base with its spray. Its action is partly mechanical, partly chemical. If then its erosive power be considerable (as when the stream is unusually silty), or if the rock yield slowly to meteoric agents (such as rain and frost), it will cut a gorge, as is commonly the case with glacier streams, and especially in crystalline rocks.

If, however, the water be generally pure, and the rock be easily affected by these other agents, the cliff will recede very slowly and uniformly, as at the Staub-Bach, near Lauterbrunnen, or, at most, will be modified into a crescentic hollow, as at the Nant Dant, near Samoens.

If, then, we have a number of small streams acting in this way, we shall get a cliff formed which will be either a straight wall or, since the streams towards the middle will probably be larger and more nearly perennial than those at the side, an amphitheatre.

The instances to which I have referred have exhibited this process on a grand scale ; the annexed diagram (fig. 3) exhibits the same on a small one. It is taken from a little cirque or, rather, a recess

appear to me of doubtful value. The vast quantity of mud and silt borne down by the stream from a glacier is adduced as a proof of its destructive power ; and it is argued " so many cubic yards of silt deposited annually, so many of rock removed annually from the bed of the glacier" But is this the fact ? There is often abundance of fine gravel on the surface of a glacier, which has not come from the bed ; and I am convinced that a very large portion of the silt borne out by the stream does not come from the subjacent rock, but from the blocks which, after being detached by frost, &c. from the mountains on either side, fall in vast quantities upon the surface of the glacier ; many of these are engulfed on their downward passage, and must be crushed to powder in that gigantic mill, of which the quern is rock, the rubber ice.
 * Two arguments often advanced to show the excavating power of glaciers

Again, it is assumed tacitly that as the glaciers were once enormously greater than at present, their erosive powers would be proportionately greater. But this may not be hastily assumed. The erosive effect of a glacier on its bed must surely depend far more on the grittiness of its under surface than on the weight of the ice. A child with a bit of sandpaper would scour rust away from iron faster than a man with a smooth rubber. Now it must be remembered that the more the hills are covered with snow, and the valleys filled with ice, the fewer peaks will project, and the less rock can fall upon the glacier's surface ; and so its very bigness will make it the worse file. I believe this remark to be especially applicable to a country like Scandinavia. The scouring power of clean ice cannot be great ; for Mr. Hopkins determined experimentally that the coefficient of friction of solid ice upon rather rough sandstone was tan 20°, the same as that of polished marble.