200 Prof, Forbes on Ice and Glaciers. 



The difficulty, which until 1846 did not become thoroughly 

 plain to me, was this : — How is the structure of glassy ice in- 

 duced in the glacier without previous fusion and fresh congela- 

 tion ? In 1843 I believed that the plates of hard blue ice, 

 which by their interposition between spaces of a less compact 

 character, compose the Veined Structure, could only be due to 

 the freezing of water in the glacier. I considered them as veins 

 of infiltrated water frozen during winter. To the same cause I 

 then attributed the conversion of the granular mass of the neve 

 into glassy ice. It was always a reluctant admission on my 

 part : for the congelation of infiltrated water to any great extent, 

 even in winter, was a relic of the Dilatation theory of glacier 

 motion ; and against that theory I had entered an earnest pro- 

 test. Between the printing of my Fourth Letter on Glaciers in 

 1842, and the publication of my 'Travels ' in 1843, 1 had already 

 abandoned the use of this reluctantly admitted fact — the winter 

 congelation of the depths of the glacier — so far as I had em- 

 ployed it to explain the recovery of the level of the glacier be- 

 tween autumn and the following spring ; and I felt my position 

 to be stronger when I obtained another explanation *. A simi- 

 lar but more effectual deliverance from a felt difficulty arose in 

 1846, when I arrived at a clear persuasion of what (as will ap- 

 pear from a comparative study of my writings) had already 

 dawned upon me some time previously, namely, that the glassy 

 structure of ice is attainable by the cohesion under pressure 

 (especially if aided by motion with friction, or kneading) of the 

 semi-opake and porous material of the glacier. 



Although I had previously expressed several times my grow- 

 ing belief that the renewed cohesion of the bruised surfaces of 

 the ice in the glacier proper, under the mutual pressure of their 

 parts, might account for the facts both of motion and of struc- 

 ture there, I had not until the summer of 1846 disembarrassed 

 myself of the complication arising from the conversion of gra- 

 nular snow in the neve into pellucid ice. I had, indeed, come 

 veiy near to it ; for already, in 1844, I had approximated the 

 two ph?enomena in the following passage, in which, after speak- 

 ing of the differential motion which tends to produce the veined 

 structure in the glacier proper, I added — " I believe that it is 



Prof. Forbes does not use a word which would lead us to suppose that he 

 wished to modify that assumption.— Phil. Mag. 1845, vol. xxvi. p. 354. — 

 J.T.] 



* ' Travels,' 1st ed. (1843), p. 384, or 2nd ed. p. 386; and note the 

 limitations imder which congelation in the interior of the glacier is ad- 

 mitted, at pages 232, 360, 372 of both editions. In so far as the glaeifica- 

 Hon of the neve is concerned, it was no assumption of mine. I accepted 

 the imivcrsal opinion of the time as stated by Dc Saussure, as well as by 

 MM. de Charpenticr and Agassiz. 



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