and Motion of Glaciers. 385 



Charmoz to a height of about 1000 feet above the level of the 

 glacier, and favoured by the peculiar light of the hour, observed 

 " a series of ueai'ly hyperbolic brownish bands on the glacier, 

 the curves pointing downwards, and the two branches mingling 

 indiscriminately with the moraines." The cause of these bands 

 was the next point to be considered, and his examination of them 

 satisfied him " that the particles of earth and sand and disinte- 

 grated rock, which the winds and avalanches and water-runs 

 spread over the entire breadth of the ice, formed a lodgement in 

 those portions of the glacier where the ice was most porous, and 

 that, consequently, the ' dirt-bands ' were merely indices of a pe- 

 culiarly j)orous veined structwe traversing the mass of the glacier 

 in these directions." 



Professor Forbes was afterwards led to regard these intervals 

 as the mai'ks of the annual growth of the glacier ; he called the 

 dii't-bands "annual rings*," and calculated, from their distance 

 apart, the yearly rate of movement. In fine, the conclusion 

 which he deduces from the dirt-bands is, that a glacier through- 

 out its entire length is formed of alternate segments of porous 

 and of hard ice. The dirt which falls upon the latter is washed 

 away, as it has no hold upon the surface; that which falls upon 

 the former remains, because the porous mass underneath gives 

 it a lodgement. " The cause of the dazzling whiteness of the 

 glacier des Bossons at Chamouni is the comparative absence of 

 these layers of granular and compact ice : the whole is nearly of 

 uniform consistence, the particles of rock scarcely find a lodge- 

 ment, and the whole is washed clean by every shower f." "It 

 must be owned, however," says Professor Forbes, "that there 

 are several difficulties which require to be removed, as to the 

 recurrence of these porous beds." In his fifteenth letter upon 

 glaciers, and in reference to some interesting observations of 

 Mr. Milward's, he endeavoured to account for the difference of 

 structure by referring it to an annual " gush " of the ice, which 

 is produced by the difference of action in summer and winter. 

 We are ignorant of the nature of the experiments on which this 

 theory of the dirt-bands is founded, and would offer the follow- 

 ing simple explanation of those which came under our own ob- 

 servation. 



Standing at a point which commanded a view of the Rhone 

 glacier, both above and below the cascade, we observed that the 

 extensive ice-field above was discoloured by sand and debris, dis- 

 tributed without regularity. At the summit of the ice-fall the 



* " I cannot help thinking that they are the true annual rings of the 

 glacier, which mark its age like those of a tree." — Appendix to Travel^, 

 p. 408. 



t Travels, ]). 406. 



