266 Prof. Tyndall on the Conformation of the Alps. 



scoop out valleys a mile or more in depth, and yet be unable to 

 remove the main inequalities from its own channel." Assuredly a 

 glacier is competent to remove such barriers, and they probably 

 have been ground down in some cases thousands of feet. But 

 being of more resisting material than the adjacent rock, they are 

 not ground down to the level of that rock. Were its bed uniform 

 in the first instance, the glacier would, in my opinion, produce 

 the inequalities which Mr. Ball thinks it ought to remove. I 

 have recently had the pleasure of examining some of these barriers 

 in the company of Mr. Ball ; and to me they represented nothing 

 more than the natural accidents of the locality. It would, I think, 

 be far more wonderful to find the rocks of the Alps perfectly 

 homogeneous, than to find them exhibiting such variations in 

 point of resistance as are actually observed. 



The question of lake-basins is in competent hands ; and on 

 its merits I will at present offer no opinion. But I cannot 

 help remarking that the dams referred to by Mr. Ball furnish 

 a conclusive reply to some of the arguments which have been 

 urged against Prof. Ramsay's theory. These barriers have been 

 crossed by the ice, and many of them present steeper gradients 

 than Prof. Ramsay has to cope with in order to get his ice out 

 of his lake-basins. An inspection of the barriers shows that 

 they were incompetent to embay the ice : they are scarred and 

 fluted from bottom to top. When it is urged against Prof. 

 Ramsay that a glacier cannot drop into a hole 2000 feet deep 

 and get out again, the distance ought to be stated over which 

 these 2000 feet have to be distributed. A depression 2000 feet 

 deep, if only of sufficient length, would constitute no material 

 obstacle to the motion of a great glacier. With a suitable pres- 

 sure from behind, the glacier would assuredly scrape along its 

 bed. The retardation of a glacier by its bed is often referred to 

 as proving its incompetence 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 ; and to make ice slide over itself requires 

 great power. We get some idea of the crushing pressure which 

 the moving glacier exercises against its bed from the fact that 

 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 Rhone at Martigny 

 has been regarded as conclusive evidence against the theory of 

 erosion. Why, it has been asked, did not the glacier of the 

 Rhone go straight forward instead of making this awkward bend ? 

 But if the valley be a crack, why did the crack make this bend ? 



