ON THE STRUCTURE AND MOTION OF GLACIERS 23 
other spaces covered with it, but there is no appearance of regularity 
in this distribution. At some places large irregular patches appear, 
and at others elongated spaces covered with dirt. Towards the 
bottom of the cascade the aspect changes ; but still, were the eye not 
instructed by what it sees lower down, the change would have no 
significance. When the ice has fairly escaped from the gorge, and has 
liberty to expand laterally in the valley below, the patches of dirt are 
squeezed by the push behind them, and drawn laterally into narrow 
stripes, which run across the glacier; and as the central portion 
moves more quickly than the sides, these strips of discoloration form 
curves which turn their convexity downwards, constituting, we 
suppose, the “ Dirt-Bands” of Professor Forbes. On the Grindelwald 
glacier, where one of us, in his examination of the bands, was accom- 
panied by Dr. Hooker, this change in the distribution of the dirt,— 
the squeezing, lateral drawing act, and bending of the dirt patches 
below the bottom of the ice-fall, was especially striking. 
Such then appears to be the explanation of the dirt-bands, in the 
cases where we have had an opportunity of observing them. We 
have not seen those described by Professor Forbes, but the conditions 
under which he has observed them appear to be similar. An illustra- 
tion of the explanation just given is furnished by the dirt-bands 
observed below the “cascade” of the Talefre. The character of this 
ice-fall may be inferred from the following words of Professor Forbes, 
and from the map which accompanies his ‘ Travels.’ “ The structure,” 
he says, “ assumed by the ice of the Taléfre is extirpated wholly by its 
precipitous descent to the level of the Glacier de Léchand, where it 
reappears, or rather is reconstructed out of the broken fragments 
according to a wholly different scheme.” One of the results of this 
“scheme” would, it is presumed, be a redistribution of the dirt, and 
the formation of bands in the manner described. Those who consult 
the map will, however, see dirt-bands marked on the Glacier du 
Géant, also, while no cascade is sketched upon it; but at page 167 of 
the ‘ Travels,’ Professor Forbes, in referring to this glacier, says, “I am 
not able to state the exact number of dirt-bands between ¢he foot 
of the ice cascade opposite La Noire and the corner of Trelaporte.” 
Here we are not only informed of the existence of a cascade, but are 
left to infer that the dirt-bands begin to form at its base, as in the 
Glacier du Géant, and in those which have come under our own 
observation. The clean Glacier des Bossons, also, which was referred 
to by Professor Forbes, in one of his earliest letters, as affording no 
lodgement to the dirt, possesses its cascade (page 181), and here also 
we find (page 182) “that the peculiar phenomena of ‘ dert-dands’ on a 
