274 APPENDIX, NO. V. 



at its lower extremity, and in contact with the ground, is scarcely 

 moving at all. 



" There is nothing of which we know less than the cause of the 

 seemingly capricious advance and retreat of the extremities of glaciers 

 at the same time, and under, seemingly, the same circumstances. 



" In the present case, I will only mention as a possible explanation, 

 that the glacier of Blaitiere probably possesses a continuous slope, from 

 its middle and higher region down to its lower extremity. But the 

 Bossons, after its steep descent from Mont Blanc, proceeds a long way 

 on a comparatively level embankment, which at an early period it 

 cast up of its own debris, and in which it has dug itself a hollow bed 

 in which it nestles. The angular slope of the bottom in contact with 

 the soil is very probably much less than in the case of the glacier of 

 Blaitiere. Now, when winter has dried up the percolating water, the 

 viscosity of the mass may be insufficient to drag it over the less slope, 

 although it carries it over the greater. That the motion of the ice 

 close to the ground should be nearly nothing, whilst the more super- 

 ficial part of the glacier over-rides it by its plasticity, is as a separate 

 fact quite in accordance both with theory and previous observation. 



" But as the snout, or lower end of the glacier of Bossons, is almost 

 stationary, whilst the middle region is moving at the rate of a foot a 

 day, Mr. Blackwell very pertinently asks, ' What becomes, then, of 

 the ice continually descending from above ? Does it not go to thicken 

 the whole mass, accumulating behind the more rigid portion below, as 

 water behind a dam ? ' I answer, undoubtedly ; and he will find this 

 explanation given ten years ago in my Travels in the Alps (2d edit., 

 p. 386). Speaking of the superficial waste of the glaciers in summer 

 and autumn, and the manner in which it is repaired before the ensuing 

 spring, I there observed, ' The main cause of the restoration of the 

 surface is the diminished fluidity of the glacier in cold weather, which 

 retards (as we know) the motion of all its parts, but especially of those 

 parts which move most rapidly in summer. The disproportion of 

 velocity throughout the length and breadth of the glacier is therefore 

 less, the ice more pressed together, and less drawn asunder ; the cre- 

 vasses are consolidated, while the increased friction and viscosity 

 causes the whole to swell, and especially the inferior parts, which are 

 the most wasted." (See also Seventh Letter on Glaciers.* 



* [Page 60 of this volume.] 



