THE STEUCTIJEE AND MOTION OF GLACIEES. 
348 
about 1000 feet above the level of the glacier, and, favoured by the peculiar light of the 
hour, observed “ a series of nearly 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 disintegrated 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 'peaudiarly ])orous veined 
structure traversing the mass of the glacier in these directions^ 
Professor Foebes 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 suiTace ; 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 com- 
parative 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 showerf .” “ It must be owned, however,” says Professor Foebes, 
“ 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. Milwaed’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 fol- 
lowing simple explanation of those which came under our own observation. 
Standing at a point which commanded a view of the Phone glacier, both above and 
below the cascade, we observ^ed that the extensive ice-field above was discoloured 
by sand and debris, distributed without regularity. At the summit of the ice-fall the 
valley narrows to a gorge, and the slope do\vnwards 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 brealdng 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 du’t, and other spaces covered with 
it, but there is no appearance of regularity in this distribution. At some places large 
* “ 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 Travels, p. 408. 
t Travels, p. 406. 
