1846.] VELOCITY OF SURFACE AND BOTTOM COMPARED. 173 



the glacier is of great thickness, the upper part of it (to which 

 alone we have access) may be expected to move uniformly or 

 sensibly so, which accounts for the approximate vertically of 

 most crevasses, during the limited period of their existence.* It 

 is, therefore, near the contact of a glacier with the subjacent 

 soil, that the most sensible effect may be looked for. The cir- 

 cumstances which first suggested themselves to me as the most 

 favourable for such an observation, were where a glacier emerg- 

 ing from a gorge, or from between a double mound of its own 

 formation, falls into a valley, and presents, for some space, lateral 

 faces of ice, not, indeed, quite vertical, but still very highly in- 

 clined, and which repose immediately on a bed of rubbish, which, 

 if not flat, but sloping somewhat towards the centre of the glacier, 

 might be considered, beyond cavil, as the floor or bed on which 

 it rests. But a careful examination of several glaciers with a 

 view to such an experiment convinced me that, even if success- 

 ful, it would not be conclusive. For it almost invariably hap- 

 pens, under these circumstances, that the glacier being no longer 

 confined laterally, tends to scale off by means of fissures parallel 

 to its length (as in Plate VI. fig. 2) ; and even if these fissures 

 do not give rise to a sensible sliding of the surfaces, they indicate 

 the direction of the twist to which the ice is exposed by the more 

 rapid motion of the centre. To avoid misapprehension, I here 

 repeat that such a tendency to scale by means of longitudinal 

 fissures, occurs only where lateral compression is wanting, and 

 there, consequently, the veined structure is always feebly de- 

 veloped. 



I succeeded, however (though not without difficulty), in 

 establishing points of observation in the terminal face of the 

 Glacier des Bois, at Chamouni, whose relative position will be 

 understood from the sketch of a front view of the glacier, fig. 

 3, and from the vertical section parallel to the length of the 

 glacier in fig. 4. It will be seen by fig. 3, that whilst the 

 lateral parts of the glacier were fissured and scaly, in conse- 

 quence of the action described in the last paragraph, the central 



* [See page 55 above.] 



