ON THE STRUCTURE OF GLACIER ICE 489 



posed of larger or smaller granules of exceedingly irregular form.^ 

 separated by very obvious fissures, but nevertheless so fitted into one 

 another as to cohere with some firmness. The distance to which the 

 fissures extend into the interior of the glacier (and hence the thickness 

 ■of the superficial layer) varies a good deal ; 7 or 8 inches is perhaps 

 rather above than below their average depth ; but however this may 

 be, the important fact is, that whenever you clear away the superficial 

 layer, you find beneath it what I have termed " deep " ice — that is, ice 

 in which neither fissures nor gi'anules are discernible ; ice which tends 

 to split parallel to the veins, and shows no disposition to break up 

 into the angular fragments so characteristic of the superficial layer. 



It has been said that mere optical examination is insufficient to 

 ■disprove the existence of fissures in the deep ice, and that such fissures 

 are present, though invisible in consequence of being filled with 

 water. I have already shown that the line of contact of water and 

 ice is optically well marked, and that there is every reason to believe 

 that even the finest fissures would be visible under a sufficient magni- 

 fying power ; but those who maintain the porosity of glacier ice, rest 

 chiefly on the results of experiments made with coloured fluids. It is 

 said that glacier ice becomes infiltrated throughout its substance with 

 extreme readiness, the coloured liquid traversing fissures which are 

 more particularly developed in the course of the blue veins. It 

 became necessary, therefore, to repeat these infiltration experiments ; 

 and for this purpose, as you will recollect, I made use of the logwood 

 infusion which you had prepared, and which by its combined clearness 

 and intensity of colour was excellently fitted for the object in view. 



If a little of the infusion were poured upon the natural surface of 

 the glacier, it immediately soaked in, spreading itself in all directions 

 between the granules (but more rapidly, as I often observed, in 

 directions parallel with the veins), and staining the whole thickness 

 of the superficial layer. Whatever quantity might be poured on to 

 the surface, however, it penetrated no further than the superficial 

 layer (unless there were some obvious crack in the deeper ice) ; and 

 when the latter was cleared away with the axe, and the surface of the 

 deep ice washed or even carefully rubbed with the hand, not a trace 

 ■of the infusion could be found in it. 



If a piece of the deep ice containing several blue veins were 

 allowed to soak in the logwood infusion until it nearly melted away, 

 it remained unstained, and either wiping it or passing it quickly 

 through clean water rendered it perfectly clear and stainless. 



' The superficial layer is particularly well described by the Messrs. Schlagintweit in their 

 Untersuchungen iiber die physikalische Geographie der Alpen, 1850. 



