GEOLOGY. 297 



neve is converted into the compact ice of the glacier proper? For pome 

 time Professor Forbes, sharing the then universal opinion that snow could 

 not pass into pellucid ice without being first melted and then fro/en, was 

 inclined to attribute the conversion to the liquefaction and subsequent con- 

 gelation of the neve snow. This view is plainly open to the same objection 

 that is urged against the dilatation theory of Do Charpenticr; viz., that 

 there is every reason to believe that the cold of the most prolonged winter 

 penetrates to a comparatively small extent into the interior of the glacier. 

 Accordingly we find that, in 1840, Professor Forbes abandoned this opinion, 

 and expressed his belief that the snow is converted into ice by intense pres- 

 sure exerted upon it when it is softened by the imminent approach of the 

 thawing state; "the very first effect of which is to annihilate the strata of 

 the ne've, the most rapid glacification being effected by the kneading and 

 working of the parts upon one another, by the differential motions which the 

 semi-fluid law of glacier progression occasions, and which also necessarily 

 takes place under intense pressure." Professor Tyndall, who verifies his 

 conclusion by actual experiment, also holds that the phenomenon is caused 

 solely by intense pressure; though, of course, he does not admit to any share 

 in it that differential motion of particles which, according to Professor 

 Forbes, constitutes the glacier a viscous fluid. We may, therefore, assume 

 it to be established that the glacification of the neve' is effected by pressure. 

 We now come to the second point, the structure of glacier ice. In all glaciers 

 whose ice is well consolidated, more especially in their middle and lower 

 portions, the ice is found to be composed of alternate veins or laminae of 

 white, porous, and blue compact ice. These veins are of very varying width; 

 they are most distinct in those parts of the glacier which are subjected to 

 the greatest pressure. Their direction also varies considerably; but they 

 appear generally to traverse the whole width of the glacier, in a curve bend- 

 ing down from the sides to the centre," and dipping forward in the direction 

 of the glacier's motion. This veined or ribboned structure was first observed 

 by M. Guyot, in 1838; but the first to attach to it a theoretical significance 

 was Professor Forbes, who noticed it in the Aar glacier in 1841. He ascribes 

 its formation to the differential motion of the glacier particles, occasioned 

 by the friction of the glacier on its sides and bed, and by the pressure of the 

 upper regions on those below ; the resiilt of such motion being the separa- 

 tion of the ice into a multitude of fissui-es, which constitute the blue veins, 

 being filled up, not, as he at first believed, by the congelation of infiltcred 

 water, but simply by the effects of time and cohesion. Professor Tyndall, 

 on the other hand, conceives that these veins are the result solely of ex- 

 ternal pressure (as opposed to differential motion of particles); and he 

 states that they are always developed in directions perpendicular to the di- 

 rection of pressure. Observing that the veined ice may 1)3 generally split in 

 the direction of its laminae, he traces an analogy between the veined struc- 

 ture and the lines of cleavage in slate; and he proves by experiment that 

 wax, or any body not strictly homogeneous in its structure, may be en- 

 dowed, by pressure, with the property of cleaving in lines perpendicular to 

 the direction of the pressure. He further proves by experiment that ice, 

 when subjected to pressure, is liquefied in lines perpendicular to the direction 

 of pressure. 



Notwithstanding the extreme beauty and ingenuity of Professor Tyndall's 

 experiments, we cannot think that they are capable of affording a complete 

 explanation of the veined structure of ice. As far as the development of 



