64 



REV. A. IEVING ON THE 



a glacier-mass which is used up within the glacier. Now for the 

 bearing of this upon the question of excavation. 



Exactly so far as the glacier-mass possesses this yielding property 

 due to the transformation within it of the mechanical energy due to 

 its weight, is its digging- or excavating-power diminished. Further, 

 whatever theory we adopt to account for the yielding property of ice 

 (its " plasticity " or " Nachgiebigkeit "), it is plain that the forward 

 and downward thrust is not wholly expended in propelling the 

 glacier mass as a rigid whole ; hence the absence of any traces of 

 excavating-action where glaciers have receded in recent years. As 

 it is, the forward thrust is to a very large extent resolved into an 

 indefinite number of smaller forces, which are expended, either 

 directly or indirectly (if first transformed into thermal energy), in 

 overcoming cohesion. It follows at once from this, as a simple de- 

 duction from the law of the conservation of energy, that the residuum 

 of energy available for any supposed excavating action of a glacier is 

 comparatively small. And this deduction would seem equally sound 

 whether (following Tyndall and Helmholtz, as I have done above) 

 we adopt the r eg elation-theory, or the " viscous theory" which was 

 propounded by Forbes, to explain the " flow " of the glacier. The 

 essential point is, that the greater part of the forward thrust of the 

 glacier mass is expended in overcoming cohesion and in causing 

 movements among the parts of a glacier relatively to one another. 

 Such relative movements of the parts of a glacier are, since the 

 numerous observations of Tyndall and his fellow glacialists, too 

 well known to need further description here ; the relative rates (1) 

 of the middle and the sides, (2) of the top and bottom, having been 

 made matters of exact measurement *. Tyndall and Helmholtz have 

 both also given experimental demonstrations of them. 



The above reasoning applies of course to such portions of the 

 glacier as form a continuous whole. There is yet another way in 

 which some part of the potential energy due to weight is expended ; 

 that is, in the formation of crevasses. Ice is not viscous, and there- 

 fore does not preserve its continuity under the influence of tensile 

 strain. So small is its power to resist tensile force, that the slight 

 bending of its mass which is caused (according to Helmholtz) by an 

 increase of gradient in its bed of from 2° to 4° is enough to form 

 transverse crevasses in its upper surface. Such crevasses penetrate 

 further into the ice in proportion as the increase of gradient is 

 greater. Here then is an expenditure of a portion of the weight of 

 the ice-mass immediately below each crevasse, which is quite un- 

 available for purposes of erosion. Again, in the formation of the 

 well-known Bergschrund, the ice below it having torn itself away 

 from the neve above it, the weight of the latter is no longer capable 

 of cooperating with the weight of the ice below it. Marginal cre- 

 vasses result also from the same absence of ductility in ice. The 

 movement forwards and downwards of the central parts of a glacier 

 being greater than that of the lateral parts, which are retarded by 

 friction against the sides of the valley, a strain and tear result ; 

 * Vide ' Forms of Water,' by Prof. J. Tyndall. 



