158 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
generally entertained would have been conclusive against the hypothesis of plasticity 
called forth by the gravity of the mass. 
So far, then, as appears from his writings, De Saussure considered the ice of gla- 
ciers to constitute a mass possessing rigidity in the highest degree, such rigidity in 
short as common experience assigns to ice tranquilly frozen in small masses, which 
is sensibly inflexible. It is in this sense in which I have spoken of De Saussure’s 
sliding theory, as one which “ supposes the mass of the glacier to be a rigid one 
sliding over its trough or bed in the manner of solid bodies*,” and I adhere to the de- 
flnition as excluding the introduction of the smallest flexibility or plasticity, to which 
the term rigidity is correctly opposed. I consider too that De Saussure’s theory 
supposes the mass of the glacier to slide over its trough or bed in the manner of 
solid bodies, that is, not as a heap of rubbish or absolute fragments, such as a gla- 
cier sometimes precipitates over a rock, but which evidently did not enter into De 
Saussure’s explanation, nor, in fact, required any theory. 
As to the crevasses which form so prominent a feature of many glaciers (although 
many are in parts almost devoid of them), Ido not recollect thatDE Saussure alludes 
to them as facilitating in any way the movement of the glacier, but simply as results 
of its motion and of the rigid character of ice. And I believe that this view (whether 
it was held by De Saussure or not) is substantially correct. The regular system of 
crevasses of a glacier is approximately transverse, rather arched upwards towards the 
origin of the glacier, and as De Saussure supposes the glacier to be pressed down- 
wards by the mass of snow accumulating at its head, it is hard to believe that he 
could have regarded these fissures as in any way essential to its movement, even were 
they very numerous ; the tendency of such a pressure from above would rather seem 
to be to pack the ice like an arch, opposing its convex side to the direction of the 
pressure. 
The view now given of De Saussure’s theory of glacier motion is not only con- 
formable to what may be gathered from his writings, but expresses the unanimous 
understanding of his numerous commentators, followers and opponents. As some 
doubt has lately been hinted as to the definiteness of De Saussure’s conception of a 
glacier as a mass devoid of flexibility and plasticity and urged down a slope as a whole, 
by the lubricating action of fusion in contact with the soil to an extent which, in ex- 
treme cases, might even give it the character of buoyancy, I will take the liberty of 
quoting some indisputable authorities amongst writers of name in different countries. 
And first from De Saussure himself: — 
“ La chaleur de la terre fait fondre les neiges et les glaces, meme pendant les 
froids les plus rigoureux lorsque leur epaisseur est assez grande pour preserver du 
froid exterieur les fonds sur lesquels elles reposent.” — Voyages, § 532.*** “C’est 
elle qui entretient les torrents, qui, meme pendant les plus grands froids, ne discon- 
tinuent jamais de sortir de tons les grands glaciers.” § 533. *** 
* Travels, p. 362. 
