Prof. Forbes on Ice and Glaciers. 201 



during the progress of the glacier thus subjected to a new and 

 peculiar set of forces depending upon gravity, and which re- 

 model its internal constitution by substituting hard blue ice in 

 the form of veins for its previous snowy texture, that the hori- 

 zontal stratification observed in the higher part of the glacier or 

 neve is gradually obliterated*." You see that even at this 

 date the formation of the hard blue ice of the veined structure, 

 and the conversion of the neve into pellucid ice, appeared to me 

 to be very intimately connected, — so much so, that until the 

 last could be explained without interior congelation, little was 

 gained in point of simplification by rejecting it in the former 

 instance. It will be found, as I have said, by a consecutive 

 perusal of what I wrote between 1844 and 1846, how gradually 

 I approached the conviction (only obtained on the spot in the 

 latter year) that the phsenomena might be all explained by 

 pressure and cohesion, the latter arising from the softening of 

 snow or ice when thaw is imminent or in progress. In 1846, 

 then, I abandoned no part of the theory of the veined structure 

 on which, as you say, so much labour had been expended, ex- 

 cept the admission, always yielded with reluctance, and got rid 

 of with satisfaction, that the congelation of water in the crevices 

 of the glacier may extend in winter to a great depth f. 



I remain, dear Sir, 

 Prof. Tyndall, Yours faithfully. 



Royal Institution. James D. Forbes. 



P.S. — It is foreign to the purpose of this letter to express an 

 opinion as to whether any theory which you have formed as to 

 the "veined structure" is independent, or otherwise, of the 

 fundamental idea of differential motion arising under conditions 

 of unequal pressures combined with friction. I reserve it until 

 your matured views are published. But it did appear to me, 

 some time ago, that in Mr. Sorby's most ingenious researches on 

 slaty cleavage, and in your extension of them to homogeneous 

 bodies such as wax and dough, the efficiency of difi"erential 

 motion was tolerably apparent. — J. D. F. 



* Sixth Letter on Glaciers, 1844. 



t That this is a question of some nicety will he readily perceived ; and 

 my caution in overtly rejecting its influence altogether may be understood. 

 For winter congelation must act to some de])th ; and certain i)ha;nomena 

 cannot be explained without it. Such are the lenticular frozen cavities 

 which you have described in your earliest paper on glaciers, which I take 

 to be the same as those longitudinal intiltrated cracks previously described 

 by me as visible iu the glacier of Hossons, and other torrential glaciers. 

 Such icy infiltrations are apparently too wide to be produced exce])t by 

 direct cougelation. But then they are always near the side or surface of 

 a glacier. 



Phil. May. S. 4. Vol. 17. No. 113. March 1859. V 



