246 Mr. E. D. Oldham on the Modulus of Cohesion of Ice. 



so that it would require a pressure of over 9 miles of ice to 

 force a glacier en masse through and out of the Lake of 

 Geneva. Compare this with the observed modulus ; and further 

 comment is superfluous. 



Having thus proved that, in the case of the Lake of Geneva, 

 the theory of glacial erosion is inadmissible, it may be well to 

 show what is the very largest lake-basin that could possibly 

 be scooped out by a glacier. To this end let both 6 and /3= 5°, 

 a supposition more favourable than is found to be the case in 

 nature ; then by (VII.), 



J) _m±z-y_ 

 2^ cot 5° 



Here, taking y as \z and fi as *2, we get the result that a gla- 

 cier, 5000 feet in depth at the head of a lake-basin and thinning 

 off to 2500 feet at its base, could not scoop out a lake of more 

 than 700 feet in depth under any circumstances whatever, nor 

 indeed could it scoop out one of even that depth; but I am at 

 present only attempting to find a limit to its power. 



One more point. The greatest distance to which a glacier 

 could be forced en masse is given by (X.) as 



T m + i — l 



Here, taking i as 5000 feet and I as nothing, we get L = 30,000 

 feet, or rather over 5 miles ; so that a glacier debouching 

 on a plain could not exert any erosive power on that plain for 

 more than five miles from the commencement of its level 

 course, and consequently could not scoop out a lake-basin of 

 more than that length, whatever its depth might be, nor could 

 it be pushed over a plain en masse for more than that distance ; 

 but if it did extend further, this could only be possible by the 

 sliding of the upper over the lower portions of the glacier, by 

 virtue of the pseudo-fluidity of ice. 



These last figures also show the fallacy of the idea that a 

 vast ice-cap*, such as is supposed by some to have covered 

 the greater part of Scotland, Ireland, and Wales, and even to 

 have extended continuously to Scandinavia, could move en 

 masse over distances measured, not by miles, but by hundreds 

 of miles, passing in its onward career over hill and valley, 

 mountain and plain, with one general movement 'of its own, 

 totally independent of the shape of the ground over which it 

 moved, and everywhere polishing and scratching the rocks 

 over which it passed in one general direction — that of its own 



* I owe this suggestion to Mr. R. Mallet ; F.R.S. 



