1885.] 



and the Motion of Glaciers. 



107 



fixed parts of the glacier. It appears that the snow or neve above 

 the " Bergschrund " does not move downwards as a glacier, but that 

 the snow which falls upon it slides down from time to time in ava- 

 lanches over the surface, usually choking up the "Bergschrund" in 

 the spring. On the other hand, everything below the " Bergschrund " 

 moves forward, slowly indeed but still moves forward, with the glacier. 

 There is, however, no marked difference, at any rate in the'conditions 

 of the surface layers, between the snow or neve above and below the 

 " Bergschrund," so that it seems at first sight paradoxical that the 

 neve on the steep slope should remain at rest while that on the less 

 steep slope below is in motion. I believe the essential difference to 

 be in the temperature of the lowest stratum which is in contact with 

 the rock. When the temperature is 0° 0. the neve is soft and vis- 

 cous, and therefore yields gradually even to a very moderate pres- 

 sure. The forces acting upon the lower stratum of neve in the upper 

 and steeper portions are greater, but the neve in contact with the 

 rock is hard frozen and unyielding. No doubt the neve above the 

 "Bergschrund" is usually of comparatively small depth, but even a 

 comparatively thin layer of neve will advance slowly in true glacier 

 fashion when it rests upon a gently sloping bed in the hollow of a 

 mountain side, and so receives enough central heat to keep the lowest 

 stratum in a soft condition. This is well seen in some of the so-called 

 glaciers of the second order. 



If the regions of neve below 0° C. which have been previously 

 spoken of exist in the moving portion of the glacier, they may not 

 improbably move on with very little change of form, riding as it 

 were upon the warmer and softer substance below. Before they can 

 reach the lower part of the glacier and be transformed by regelation 

 into compact ice their temperatures must have risen to 0° C, and they 

 cannot again fall below this temperature except close to the surface of 

 the glacier. 



VII. Concluding Remarks. 



The general result of the foregoing paper seems to be that the 

 fuller consideration of the physical properties of glacier ice leads to 

 essentially the same conclusions as those to which Forbes was led 

 forty years ago by the study of the larger phenomena of glacier 

 motion — that is, that the motion is that of a slightly viscous mass 

 partly sliding upon its bed, partly shearing upon itself under the in- 

 fluence of gravity. To say this is, however, by no means to deny 

 the importance of regelation in the economy of a glacier. To rege- 

 lation mainly we must attribute the gradual passage of snow through 

 the form of neve into ice, the healing of crevasses, and the possibility 

 of comparatively rapid and violent changes of form in portions of a 



