337 



and a portion of the glacier where they did not exist was presented 

 to us simultaneously with the bands upon another portion. Their 

 proximate origin and final completion were thus before us. The 

 explanation oifered in the paper is, that the dirt scattered by ava- 

 lanches and winds over the upper regions of a glacier is redistri- 

 buted by the passage of the glacier down a cascade where the ice is 

 shattered, and the dirt broken up into detached patches. On reach- 

 ing the bottom, where the force becomes one of compression, the 

 patches of dirt are squeezed longitudinally and drawn out laterally, 

 being thus converted into stripes of discoloration, which, owing to 

 the speedier motion of the centre of the glacier, are convex towards 

 its lower extremity. On consulting the map of Prof. Forbes, it will 

 be seen that the dirt-bands commence at the base of the ice-fall of 

 the Talefre, while none exist above the fall. Those shown on the 

 Glacier du Ge'ant, we are led to infer, commence at the base of the 

 Cascade of la Noire, which, however, is not sketched on the map. 

 The theory of Prof. Forbes is, that a glacier, throughout its entire 

 length, is composed of alternate segments of hard and porous ice ; 

 that the dirt is washed from the former, but finds "a lodgement" in 

 the latter, thus giving rise to the phenomenon of the bands. We 

 are unacquainted with the experiments on which this theory is 

 founded, and have only to state, that the bands which we have ob- 

 served seem accounted for by the simple explanation given above. 

 In the paper, experiments are described illustrative of this view, and 

 in which the bands are accurately exhibited on a small scale. 



In the course of the inquiry much assistance was derived from the 

 use of troughs of various shapes and sizes, through which mud, 

 formed by the mixture of finely-sifted pipeclay and water, was per- 

 mitted to flow. Coloured circles being stamped upon the mud, 

 from the distortion of these the character of the forces, whether 

 compression or tension, acting upon the mass was inferred. It is 

 needless to say, that this mode of experiment does not pledge the 

 authors to the view that ice is similarly constituted to mud. "Where 

 the mud stretches, the ice breaks ; and the experiments were mainly 

 instituted for the purpose of examining the relation of the forces act- 

 ing upon a glacier to its directions of fracture. 



It may, however, be urged, that, after all, the foregoing experiments 

 do not prove the viscous theory to be wrong. The mere fact of 



