146 VISCOUS THEORY OF GLACIER MOTION. [1846. 



occasions a convexity of the lines of fissure towards the origin 

 of the glacier. Opposite Montanvert the crevasses form two 

 systems inclined 65 to one another, but this appears to be a 

 casual occurrence arising from a fresh strain being imposed on 

 the ice owing to its rigidity when the direction of the bed or 

 trough suddenly changes, and the two-fold systems probably 

 coexist but for a short space, one tending to close whilst the 

 other opens. Be this as it may, unless where a glacier is 

 falling headlong in the manner of a cascade, the crevasses do 

 not produce any actual dislocation of its mass into blocks or 

 fragments, since the crevasses rarely intersect even where most 

 numerous, but almost invariably thin out in the solid mass 

 whilst another crevasse takes its origin a little to one side or 

 other leaving a firm connection of ice between them ; and the 

 difficulty and danger of traversing a glacier where [it is] much 

 crevassed, does not arise from the necessity of leaping from square 

 to square of ice, but from having to traverse these bridges of 

 icy communication which even there link the glacier together, 

 and which are almost always sharp on their upper edge when 

 the season of the year is pretty far advanced, owing to the 

 continual dripping. 



The occurrence of crevasses which cut up a glacier into 

 square or trapezoidal blocks, is sufficiently infrequent to deserve 

 notice. Such occur when a glacier of the second order de- 

 scends over a boss of granite, or a surface convex in all 

 directions. We have then radiating crevasses combined with 

 concentric ones, producing a tartan-like appearance. Such 

 may be seen in a glacier of the second order on the south side 

 of the Aiguilles of Charmoz and Grepon, above the Glacier du 

 Geant ; and it is a very convincing proof of the essential tenacity 

 of a glacier, that, with a surface so scarred and intersected, the 

 fragments do not fall away in avalanches. This only is to be 

 explained by the consideration that, thin as are the glaciers of 

 the second order, the apparent dislocation is only superficial.* 



* [When they fall in avalanches, or are about to do so, on the verge of a pre- 

 cipice, the detached trapezoidal blocks received from De Saussure (or rather from 



