386 Professors Tj^ndall and Huxley on the Sb-ucture 



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 ai'e observed quite free from dirt, and 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 bot- 

 tom 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 stripes of discoloration form curves which turn their con- 

 vexity 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 accompanied by Dr. 

 Hooker, this change in the distribution of the dirt, — the squeez- 

 ing, 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 whei'e 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 illustration of the explanation just given is fur- 

 nished 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 Talefre is extirpated wholly by its precipitous 

 descent to the level of the Glacier de Lechand, where it reap- 

 pears, 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 Geant 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 the foot of ihe ice cascade opposite La Noire 



