ALPINE SCULPTURE. 245 



this amount the glacier is urged down its valley by the 

 pressure from behind. We can hardly, I think, deny 

 to such a tool a power of excavation. 



The retardation of a glacier by its bed has been 

 referred to as proving its impotence as an erosive 

 agent ; but this very retardation is in some measure an 

 expression of the magnitude of the erosive energy. 

 Either the bed must give way, or the ice must slide 

 over itself. We get indeed some idea of the crushing 

 pressure which the moving glacier exercises against its 

 bed from the fact that the resistance, and the effort to 

 overcome it, are such as to make the upper layers of a 

 glacier move bodily over the lower ones a portion 

 only of the total motion being due to the progress of 

 the entire mass of the glacier down its valley. 



The sudden bend in the valley of the Khone at 

 Martigny has also been regarded as conclusive evidence 

 against the theory of erosion. 'Why,' it has been 

 asked, ' did not the glacier of the Ehone go straight 

 forward instead of making this awkward bend ?' But 

 if the valley be a crack, why did the crack make this 

 bend? The crack, I submit, had at least as much 

 reason to prolong itself in a straight line as the glacier 

 had. A statement of Sir John Herschel with reference 

 to another matter is perfectly applicable here : ' A 

 crack once produced has a tendency to run for this 

 plain reason, that at its momentary limit, at the point 

 at which it has just arrived, the divellent force on the 

 molecules there situated is counteracted only by half 

 of the cohesive force which acted when there was no 

 crack, viz. the cohesion of the uncracked portion alone ' 

 ('Proc. Roy. Soc.' vol. xii. p. 678). To account, then, 

 for the bend, the adherent of the fracture theory must 

 assume the existence of some accident which turned 

 the crack at right angles to itself ; and he surely will 



