34 THE FOEMS OP WATER IN 



210. Are we to understand by this that the ice of 

 the Lechaud is squeezed to one- tenth of its former 

 volume ? By no means. It is mainly a change of form, 

 not of volume, that occurs at Trelaporte. Previous to 

 its compression, the glacier resembles a plate of ice 

 lying flat upon its bed. After its compression, it 

 resembles a plate fixed upon its edge. The squeezing, 

 doubtless, has deepened the ice. 



§ 30. Longitudinal Compression of a Glacier. 



211. The ice is forced through the gorge at Trela- 

 porte by a pressure from behind; in fact the Glacier 

 du Geant, immediately above Trelaporte, represents a 

 piston or a plug which drives the ice through the 

 gorge. What effect must this pressure have upon the 

 plug itself? Reasoning alone renders it probable that 

 the pressure will shorten the plug ; that the lower part 

 of the Glacier du Geant will to some extent yield to the 

 pressure from behind. 



212. Let us test this notion. About three-quarters 

 of a mile above the Tacul, and on the mountain slope to 

 the left as we ascend, we observe a patch of verdure. 

 Thither we climb ; there we plant our theodolite, and 

 set out across the Glacier du Geant, a line, which we 

 will call line No. 1 (F F upon sketch, p. 68). 



21 R, About a quarter of a mils lower down we find a 

 practicable couloir on the mountain side ; we ascend 



