MOTIONS OF GLACIER ICE 145 



attain e\t rat military thickness. Urn ilu- burden of movement seems to be thrown 

 almost wholly on compression, with the- slight aid of molecular changes dm- to 

 internal c\ aporation. Since the temperature in the u|>|>er part of the ice is very 

 low most Hi each year, the compression must be great before it becomes effective 

 in melting; hence tin- great thickness of ice necessary before motion is considerable. 

 Similar conditions affect the heads of Alpine glaciers, though here the high gradients 

 favor motion among the granules of ice. In the lower reaches of Alpine glaciers, 

 where the temperatures are near the melting-point, and where the ice is bathed in 

 water much of the time, movement may take place in ice which is thin and compact. 



If the views here presented are correct, there is also, at all points below the 

 source, the co-operation of rigid thrust from behind, with the tendency of the mass 

 to move on its own account. The latter is controlled by gravity, and conforms 

 in its results to laws of liquid flow. The former is a mechanical thrust. This 

 thrust is different from the pressure of the upper part of a liquid stream on the 

 lower part, because it is transmitted through a body whose rigidity is effective, 

 while the latter is transmitted on the hydrostatic principle of equal pressure in all 

 directions. Thrust would be most effective toward the end or edge of a glacier. 



Corroborative phenomena. The conception of the glacier and its movement 

 here presented explains some of the anomalies that otherwise seem paradoxical. 

 If the ice is always a rigid body which yields only as its interlocking granules 

 change their form by loss and gain, a rigid hold on the imbedded rock at some 

 times, and a yielding hold at others, is intelligible. Stones in the base of a glacier 

 may be held with great rigidity when the ice is dry, scoring the bottom with much 

 force, while they may be rotated with relative ease when the ice is wet. In short, 

 the relation of the ice to the bowlders in its bottom varies radically according to its 

 'dryness and temperature. A dry glacier is a rigid glacier. A dry glacier is neces- 

 sarily cold, and a cold glacier is necessarily dry. 



It is difficult to explain the furrows and grooves cut by glaciers in firm rock if 

 the ice is so yielding as to flow under its own weight on a surface which is almost 

 flat. If the mass is really viscous, its hold on its imbedded debris should also be 

 viscous, and a bowlder in the bottom should be rotated in the yielding mass when 

 its lower point catches on the rock beneath, instead of being held firmly while a 

 groove is cut. This is especially to the point since viscous fluids flow by a partially 

 rotary movement. 



On the view here presented, a glacier should be more rigid in winter than in 

 summer. The total thickness of a glacier should experience this rigidity of winter 

 at its ends and edges, where the ice is thin enough to permit the low temperature 

 to affect its bottom. The motion in these parts during the winter is, therefore, very 

 small. 



In this view, ak>o, may be found an explanation of the movement of glaciers 

 for considerable distances up-slope, even when the surface of the ice, as well as its 

 bed, is inclined backwards. So far does this go, that a few superficial streams 

 run for some distance backwards, i. e., toward the heads of the glaciers, while in 

 other places surface waters are collected into ponds and lakelets. Such a slope 

 of the surface of ice is not difficult to understand if the movement is due to thrust 

 from behind, or if it is occasioned by internal crystalline changes a< -ting on a rigid 

 body; but it must bo regarded as very remarkable if the movement of the ice is 

 that of a fluid body, no matter how viscous, for the length of the acclivity is in some 

 cases several times the thickness of the ice. Crevassing and other evidences of 



