GRAVITATION THEORY DILATATION THEORY. 249 



it over the obstacles opposed by friction and the irregularities 

 of the surface on which it moves. 



The following quotation from De Saussure explains his 

 views with his usual precision : " These frozen masses, carried 

 along by the slope of the bed on which they rest, disengaged by 

 the water (arising from their fusion owing to the natural heat 

 of the earth) from the adhesion which they might otherwise 

 contract to the bottom sometimes even elevated by the water 

 must gradually slide and descend along the declivity of the 

 valleys or mountain slopes (croupes) which they cover. It is 

 this slow but continual sliding of the icy masses (des glaces) on 

 their inclined bases which carries them down into the lower 

 valleys, and which replenishes continually the stock of ice in 

 valleys warm enough to produce large trees and rich harvests."* 

 Very sufficient objections have been urged against this theory. 

 It is evident that De Saussure considered a glacier as an accu- 

 mulation of icy fragments, instead of a great and continuous 

 mass, throughout which the fissures and crevasses bear a small 

 proportion to the solid portion ; and that he has attributed to 

 the subglacial water a kind and amount of action for which 

 there exists no sufficient or even probable evidence. The main 

 objection, however, is this, that a sliding motion of the kind 

 supposed, if it commence, must be accelerated by gravity, and 

 the glacier must slide from its bed in an avalanche. The small 

 slope of most glacier-valleys, and the extreme irregularity of 

 their bounding walls, are also great objections to this hypothesis. 



The Dilatation theory ingeniously meets the difficulty of the 

 want of a sufficient moving power to drag or shove a glacier 

 over its bed, by calling in the well-known force with which 

 water expands on its conversion into ice. The glacier being 

 traversed by innumerable capillary fissures, and being in sum- 

 mer saturated with water in all its parts, it was natural to 

 invoke the freezing action of the night to convert this water 

 into ice, and by the amount of its expansion to urge the glacier 

 onwards in the direction of its greatest slope. In answer to 



* Voyages dans les Alps, sec. 535. 



