JR. M. DeeJey and O. Fletcher — Structure of Glacier-Ice. 153 



where the structure was very fine-grained, the samples melted away 

 before they could be sketched. 



To the unaided eye such slides show the outlines of the crystalline 

 particles very imperfectly. When examined through the polariscope, 

 however, the whole structure is distinctly revealed. 



We do not by any means feel that, in our rather rapid survey, we 

 have been able to work out the subject with the care it demands, or 

 that in a short article full justice can be done to those who have 

 preceded us. For a full discussion see " The Glacial Nightmare," 

 by Sir Henry H. Howorth, vol. ii. p. 519. Still, as the information 

 we have already obtained seems to throw light upon the character 

 of the veined structure, and to lend some support to a theory of 

 Glacier-Motion, propounded by one of us in 1888,^ we venture to 

 put on record some of the results we have obtained. 



Until quite recently, the granular nature of Glacier-Ice has not 

 received very close attention. Hugi,* however, in 1843, remarked 

 on its granular structure, and Forbes, in a particular case, says, 

 " the mass was granular, and without structure or bands of any 

 kind," without veined structure we presume he meant. Heim, 

 McConnell, Bertin, Grad, and others have, however, by primarily 

 regarding a glacier as a crystalline aggregate, made us familiar with 

 the "Glacier grain." In a recent paper ^ Messrs. McConnell and 

 Kidd say : " Glacier-Ice is a sort of conglomerate formed of glacier 

 grains (Gletscherkorner), differing, however, from a conglomerate 

 proper in that there is no matrix, the grains fitting each other 

 .perfectly. In the winter, at any rate, the ice on the sides of the 

 glacier caves looks quite homogeneous. But when a piece is broken 

 ofi" and exposed to the sun's rays the different grains become visible 

 ,to the naked eye, being separated probably by thin films of water. 

 Though the optical structure of each grain is found under the 

 polariscope to be perfectly uniform, the bounding surfaces are 

 utterly irregular, and are generally curved. The optic axes, too, 

 of neighbouring grains seem arranged quite at random." 



Even now, however, the exact details of the process by which 

 the snow is converted into glacier ice have not been traced. Forel 

 states that he imitated the structure by alternately moistening and. 

 freezing snow. Unfortunately, during our trip, we did not examine 

 a section showing the passage of snow into compact ice. On the 

 slopes, near the summit of Mont Blanc, after a very windy, cold, 

 and somewhat snowy day and night, the snow was granular, each 

 grain being about the size of a mustard seed, a result no doubt 

 partly brought about by the freezing during the night of the snow 

 melted during the day, and partly by the condensation in the pores 

 of the snow of water-vapour. At the end of the Grand Plateau, 

 and at the base of the slope up which the way to the summit passes, 

 is a trail of debris formed of neve ice from a small secondary glacier 

 well above the snow-line. The fraerments of this ice were formed 



O 



1 Phil. Mag. February, 1888. - 



2 Die Gletscher, 1843, p. 10. 



' Proceedings of the Koyal Society, vol. xliv. p. 333. 



