322 PEOCEEBINGS OP THE GEOLOGICAL SOCIETY. [^W- 5, 



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 pro- 

 duced 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 imder the Rothstock peaks. 



Eunning 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, 

 wiU be modified into a crescentic hoUow, 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 wiU probably be larger and 

 more nearly perennial than those at the side, an amphitheatre. 



The instances to which I have referred have exhibited this pro- 

 cess 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 



* Two arguments often advanced to show the excavating power of glaciers 

 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 naust be crushed to powder in that 

 gigantic mill, of which the quern is rock, the rubber ice. 



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 re- 

 mark 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 coefiicient of friction of solid ice upon rather rough sandstone was tan 

 20°, the same as that of polished marble. 



