104 ICE AND GLACIERS. 



the levels and measurements taken by Forbes, with the view of 

 the right bank of the glacier. The letters stand for the same 

 objects as in Fig. 14 ; p is the Aiguille de Lechaud, q the Aiguille 

 Noire, r the Mont Tacul, f is the Col du Geant, the lowest point 

 in the high wall of rock that surrounds the upper end of the snow- 

 fields which feed the Mer de Glace. The base line corresponds to 

 a length of a little more than nine miles : on the right the heights 

 above the sea are given in feet. The drawing shows very distinctly 

 how small in most places is the fall of the glaeier. Only an approxi- 

 mate estimate could be made of the depth, for hitherto nothing 

 certain has been made out in reference to it. But that it is every 

 deep is obvious from the following individual and accidental 

 observations. 



At the end of a vertical rock wall of the Tacul, the edge of the 

 Glacier du Geant is pushed forth, forming an ice wall 140 feet in 

 height. This would give the depth of one of the upper arms of the 

 glacier at the edge. In the middle and after the union of the three 

 glaciers the depth must be far greater. Somewhat below the j unc- 

 tion Tyndall and Hirst sounded a moulin, that is, a cavity through 

 which the surface glacier waters escape, to a depth of 160 feet ; the 

 guides alleged that they had sounded a similar aperture to a depth 

 of 350 feet, and had found no bottom. From the usually deep 

 trough-shaped or gorge-like form of the bottom of the valleys, 

 which is constructed solely of rock walls, it seems improbable 

 that for a breadth of 3,000 feet the mean depth should only be 350 

 feet; moreover, from the manner in which ice moves, there must 

 necessarily be a very thick coherent mass beneath thecrevassed part. 

 To render these magnitudes more intelligible by refer- 

 ence to more familiar objects, imagine the valley of Heidel- 

 berg filled with ice up to the Molkencur, or higher, so that 

 the whole town, with all its steeples and the castle, is 

 buried deeply beneath it; if, further, you imagine this mass 

 of ice, gradually extending in height, continued from the 

 mouth of the valley up to Neckargemiind, that would about 

 correspond to the lower united ice-current of the Mer de Glace. 

 Or, instead of the Khine and the Nahe at Bingen, suppose two 

 ice-currents uniting which fill the Rhine valley to its upper 



