ON THE STRUCTURE AND MOTION OF GLACIERS IT 
tion, which both in his ‘ Travels’ and in a series of letters, extending 
over a period of several years, he has expounded and illustrated with 
great skill. The theory is summed up in the following words :— 
“The whole phenomena in the case of any of the semifluids I have 
mentioned (treacle, tar, &c.), are such as, combined with the evidence 
which I have given, that the motion of a glacier is actually such as I 
have described that of a viscid fluid to be, can leave, I think, no 
reasonable doubt, ¢hat the crevices formed by the forced separation of 
a half rigid mass, whose parts are compelled to move with different 
velocities, becoming infiltrated with water, and frozen during winter, 
produce the bands which we have described.” 
This theory has been opposed by Mr. Hopkins, whose excellent 
papers, published in the 26th volume of the Philosophical Magazine, 
are replete with instruction as to the mechanical conditions of glaciers. 
On the other hand, the theory of Professor Forbes is defended in the 
same journal by Dr. Whewell.2 We will leave the points discussed 
in their communications for the present untouched, and confine 
ourselves to stating a few of the circumstances which appear to us to 
render the theory doubtful. 
1. It is not certain that the colds of winter penetrate to depths 
sufficient to produce the blue veins, which, it is affirmed, are “an 
integral part of the inmost structure” of the ice. Saussure was of 
opinion that the frosts of winter did not penetrate to a greater depth 
than Io feet, even at the summit of Mont Blanc, and Professor Forbes 
considers this opinion to be a just one. But if so, there would be 
some difficulty in referring to the frosts of winter the blue veins which 
M. Agassiz observed at a depth of 120 feet below the surface of the 
glacier of the Aar. 
2. It will be remembered that M. Guyot’s statement regarding the 
blue veins is, that he saw the mass of the glacier composed of a 
multitude of layers of white ice, separated, each from the other, by a 
plate of transparent ice. The description of Professor Forbes is 
briefly this :—“ Laminz or thin plates of transparent blue ice alter- 
nate in most parts of every glacier with lamine of ice, not less hard 
and perfect, but filled with countless air-bubbles which give it a 
frothy semi-transparent look.” But there is another form of the blue 
1 Travels, p. 377. M. Agassiz also seems disposed to regard the blue bands as the result of 
the freezing up of fissures, which, however, are supposed to be formed in a manner different 
from that assumed by Professor Forbes. But M. Agassiz calls the attention of future 
observers to some of the related phenomena ; and gives it as his opinion, ‘‘ qu il n’est aucune 
phénoméne dont l’explication offre plus des difficultés.” See his important work, ‘Systeme 
Glaciére,’ which, until quite recently, we had not the opportunity of examining. 
2 Philosophical Magazine, S. 3, vol. xxvi. pp. 171, 217. 
