22 ON THE STRUCTURE AND MOTION OF GLACIERS 
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 peculiarly porous veined structure traversing the mass 
of the glacier in these directions.” 
Professor Forbes was afterwards led to regard these intervals as 
the marks of the annual growth of the glacier; he called the dirt- 
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 throughout 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 lodgement, and the whole is washed clean by every shower.” * 
“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 following simple explanation of those which 
came under our own observation. 
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 débris, dis- 
tributed without regularity. At the summit of the ice-fall the valley 
narrows to a gorge, and the slope downwards is for some distance 
precipitous. In descending, the ice is greatly shattered ; in fact, the 
glacier is broken repeatedly at the summit of the declivity, transverse 
chasms being thus formed ; and these, as the ice descends, are broken 
up into confused ridges and peaks, with intervening spaces, where the 
mass is ground to pieces. By this breaking up of the glacier the dirt 
upon its surface undergoes fresh distribution: instead of being spread 
uniformly over the slope, spaces are observed quite free from dirt, and 
? “T cannot help thinking that they are the ¢rwe annual rings of the glacier, which mark 
its age like those of a tree.” —Appendix to Travels, p. 408. 
2 Travels, p. 406. 
