162 VISCOUS THEORY OF GLACIER MOTION. [1846. 



ice descends in blocks, almost as water in a cascade often does 

 in spray, and hence, the internal strains being destroyed, no 

 structure is developed, or if previously developed, tends to wear 

 out.* 



2. In a glacier moving torrentially, that is, with frequent 

 and considerable changes of velocity, but without being divided 

 into blocks by intersecting crevasses, we find real internal cracks 

 in the ice, some feet in length, and an inch or more in thick- 

 ness, marked by the pure frozen water which fills these spaces 

 in the comparatively opaque whitish ice of which glaciers de- 

 scending rapidly from the region of the neve are composed- 

 Such are peculiarly visible in the lower and more accessible 

 region of the glacier of Bossons ; f perhaps the most instructive 

 which can be named as showing these infiltrated cracks, which 

 by their dimensions, direction, and in every other particular, 

 form a true link between the longitudinal dislocations of a tor- 

 rential glacier and the perfect veined structure or bruise into 

 which it passes by imperceptible gradations, including a perfectly 

 regular development of the frontal dip, where we might expect 

 it to be well shown, for the observations of page 128 show that 

 the lowest portion of the glacier of Bossons moves slower than 

 its middle portion ; there is therefore a manifest longitudinal 

 compression arising from the friction of the bed.J 



3. The next stage is that of the perfect bruise or veined 

 structure, best seen in the most united and least fissured parts 

 of glaciers with rocky sides and moving over a moderate slope. 

 Whatever increases lateral compression (without however ne- 

 cessitating dislocation), such as the union of two or more 

 glaciers in one, tends to develope the structure more perfectly. 

 Such cases are well seen on several parts of the Mer de Glace, 

 and of the glacier of Miage.H 



* See Third Letter on Glaciers ; page 24 of the present volume. 



f See Travels, p. 181. 



| The internal rents in the lava of Zafarana referred to in 2 of this paper, and 

 figured in Plate I., fig. 8, present a perfect analogy with those of the glacier of Bos- 

 sons, and appear to he due to the same cause. 



g Third Letter [p. 24, above.] 



|| See the figures of the structure of the glacier of Miage. Travels, p. 197. 



