Prof. A. C. Ramsay on the Glacial Theory of Lake- Basins. 287 



mouths of the valleys themselves, and still within their bounds. 

 And though Sir Charles has not met the arguments urged either 

 in my first or second paper, except by implication, I am constrained 

 by the circumstances of the case to repeat them in a manner 

 that I hope cannot be misunderstood. This may be done in a 

 very few words. Every physicist knows that when such a body 

 as glacier-ice descends a slope, the direct vertical pressure of 

 the ice will be proportional to its thickness and weight and the 

 angle of the slope over which it flows. If the angle be 5°, the 

 weight and erosive power of a given thickness of ice will be so 

 much, if 10° so much less, if 20° less still, till at length, if we 

 may imagine the fall to be over a vertical wall of rock, the pres- 

 sure against the wall (except accidentally) will be nil. But when 

 the same vast body of ice has reached the plain, then motion 

 and erosion would cease, were it not for pressure from behind 

 (excepting what little motion forward and sidewards might be due 

 to its own weight). This pressure, however, must have been con- 

 stant as long as supplies of snow fell on the mountains, and there- 

 fore the inert mass in the plain was constantly urged onwards ; 

 and because of its vertical pressure its direct erosive power would 

 necessarily be proportional to its thickness, and greater than 

 when it lay on a slope; for it would grate across the rocks, 

 as it were, unwillingly and by compulsion, instead of finding 

 its way onwards more or less by virtue of gravity. Indeed 

 the idea is forced on the mind, that the sluggish ice would 

 have a tendency to heap itself up Just outside the mouth of 

 the valley and there attain an unusual thickness, thus exer- 

 cising, after its descent, an extra erosive power. Further, as I 

 have said elsewhere, when the glacier spread well out upon the 

 plain, far beyond the mouth of the valley, it would of necessity 

 thin more and more by melting ; and this seems to me a very 

 obvious reason why, its weight being lessened, the waste of under- 

 lying matter by erosion would decrease towards what are now the 

 mouths of those lake-basins which Sir Charles, following the 

 supposition of the late Dr. Falconer, allows were filled with ice 

 during the glacial period. These propositions seem to me so 

 obvious, that I should scarcely have thought it necessary to re- 

 state them ; but if they be mere fallacies, it is singular that no one 

 has yet thought it worth while to refute them. Sir Charles himself 

 seems to allow that the ice may have had " to ascend a slope "*. 



The remark that in a " part of a valley from which a gla- 

 cier has retreated in historical times, no basin-shaped hollows 

 are conspicuous," is met, if we think of it, by the foregoing 



* To discuss the details of this subject would involve a repetition of 

 what I have already printed, and this I must necessarily avoid. Geolo- 

 gical Journal, I. n. ; Phil. Mag. October 1864, &e. 



