/ REV. A. IRVING ON THE 



conclusions adverse to the theory of excavation which has heen 

 advocated for some twenty years by Sir A. Eamsay, to whose 

 geological writings we are all so greatly indebted. 



General Conclusions. 



It will be seen from what has been advanced that I do not 

 question* the power of glaciers to do a good deal of surface-erosion 

 in grinding, polishing, grooving, and striating the rocks ; no one 

 who has seen any thing of glaciers or of glaciation could do this for a 

 moment. My contention, from the consideration of mechanical and 

 physical principles, is that far too much work has been ascribed to 

 them by some writers in the way of erosion, and that the notion of 

 actual excavation of lake-basins is inadmissible, except under very 

 special circumstances such as those under which some tarns may 

 have been formed. 



The causes of the differential movement of glaciers would appear 

 to be three : — 



(1) Cracking and partial melting in places under pressure and 

 strain, followed by regelation, as propounded by Tyndall and 

 accepted (after independent experimental investigation of the 

 phenomena) by Helmholtz. This is probably the principal regular 

 cause. 



(2) Friction generating thermal energy, and so producing lique- 

 faction, which is followed by regelation. 



These two causes, it will be seen, are in constant operation, and a 

 fortiori must have operated still more powerfully when the glaciers 

 were of much greater dimensions. 



(3) There remains to be accounted for a secondary differential 

 motion, which has, it appears, not yet received a satisfactory expla- 

 nation, though some recent writers have attempted it * : the move- 

 ment is greater (a) by day than by night, (b) in summer than in 

 winter. This was very nearly explained some years ago by Canon 

 Moseley, when he maintained that somehow or other 'radiant 

 heat ' must enter the ice. Had he known those principles of 

 physics which are illustrated by the action of Crookes's radiometer, 

 there is little doubt that he would have seen his way to the 

 right explanation. The theory which I venture here to advance 

 is based upon a series of experiments with ice subjected to 

 different sources of radiant energy, in which I have been engaged, 

 an account of which I hope to publish elsewhere f. For the present 

 purpose we must consider heat to mean energy capable of melting 

 ice or tending to melt it. Whatever notion we may attach to the 

 term " radiant heat," it is clear to me from my experiments that 

 heat, qua- heat, cannot enter the ice and be afterwards expended in 

 the work of liquefaction — that is to say, in overcoming cohesion and 

 so promoting differential movements of parts of the glacier. All 

 such heat must become latent in the liquefaction of ice at the sur- 

 face. It is in the transformation of energy that the clue is to be 



* See Oroll, ' Climate and Time,' chap. xxxi. t See Nature, No. 693. 



