254 ^^^ POPULAR SCIENCE MONTHLY. 



different manner. Its greatest action is where it is deepest in 

 the middle of the ice stream while water acts least where it is 

 deepest, and more forcibly at the side than in the middle. The 

 lake is, no doubt, deepest in the line of the old river, where the 

 valley was lowest ; and that may well have been nearer the 

 southern than the northern side of the lake. 



Another frequently urged objection is, that as the glacier has 

 not widened the narrow valley from Martigny to Bex it could not 

 have eroded a lake nearly a thousand feet deep. This seems to 

 me a complete non sequitur. As a glacier erodes mainly by its 

 vertical pressure and by the completeness of its grinding arma- 

 ture of rock, it is clear that its grinding power laterally must 

 have been very much less than vertically, both on account of the 

 smaller pressure because it would mold itself less closely to the 

 ever-varying rocky protuberances, and mainly, perhaps, because 

 at the almost vertical sides of the valley it would have a very 

 small stony armature, the blocks continually working their way 

 downward to the bottom. Thus, much of the ice in contact with 

 the sides of narrow ravines might be free of stones, and would 

 therefore exert hardly any grinding power. It is also quite cer- 

 tain that the ice in this narrow valley rose to an enormous height, 

 and that the chief motion and also the chief erosion would be on 

 the lateral slopes, while the lower strata, wedged in the gorge, 

 would be almost stationary. 



The most recent researches, according to M. Falsan, show that 

 the thickness of the ice has been usuallj^ underestimated. A ter- 

 minal moraine on the Jura at Chasseron is four thousand feet 

 above the sea, or twenty-seven hundred and seventy feet above 

 Geneva. In order that the upper surface of the ice should have 

 had sufficient incline to flow onward as it did, it was probably five 

 thousand or six thousand feet thick below Martigny and four 

 thousand or five thousand feet over the middle of the lake. It is 

 certain, at all events, that whatever thickness was necessary to 

 cause onward motion, that thickness could not fail to be produced, 

 since it is only by the onward motion to some outlet or lowland 

 where the ice can be melted away as fast as it is renewed that 

 indefinite enlargement of a glacier is avoided. The essential con- 

 dition for the formation of a glacier at all is that more ice 

 should be produced annually than is melted away. So long as 

 the quantity produced is on the average more than that melted, 

 the glaciers will increase ; and as the more extended surface of 

 ice, up to a certain point, by forming a refrigerator helps its own 

 extension, a very small permanent annual surplus may lead to an 

 enormous extension of the ice. Hence, if at any stage in its de- 

 velopment the end of a glacier remains stationary, either owing 

 to some obstacle in its path or to its having reached a level plain. 



