JR. M. Deeley and O. Fletcher — Structure of Glacier- lee. 159 



moi'e than visible traces of the direction of the internal icy 

 structure."^ Tyndall, however, has shown that the dirt-bands are 

 not " external evidence of their structure." 



Opposite the Hotel du Montanvert the dirt-bands and the veined 

 structure are almost parallel over the whole width of the glacier. 

 Higher up, however, they cease to be congruent. In all cases, as 

 far as our experience goes, the direction of the veined structure, 

 rather than being at right angles to the direction of greatest 

 pressure, was such as would be produced by the shear the glacier 

 undergoes in changing its shape, or rather the relative position of 

 its parts during its descent. The cut (Fig. 8), after Forbes,^ shows 



Fig. 8. (After J. D. Forbes.) 



fairly well the direction in which the veined structure runs. Perhaps 

 at the bottom of the ice the veins should have been shown more 

 parallel with the sides. The veins run up to the surface in the 

 middle owing to the fact that below the snow-line the surface of 

 the glacier, which is rapidly melting, exposes the edges of the shear 

 planes. Pressure alone could hardly give rise to lamination or 

 cleavage. When, however, the pressure produces a re-arrangement 

 of particles, or a change of shape, shear takes place and lamination 

 results. 



The laborious observations of J. D. Forbes and others have taught 

 us how a glacier, considered as a whole, moves. It is quite clear 

 that a glacier, as a whole, is viscous. Its river-like flow is regular 

 and continuous, and owes nothing, or nearly nothing, to the 

 formation of crevasses. And, indeed, a lump of Glacier-Ice does 

 exhibit marked viscosity. A continuously-applied pressure produces 

 permanent deformation, and it would appear that under any stress, 



1 "The Theory of Glaciers," p. 22. 



2 jjj^. p^ 247. 



