62 REV. A. IRVING ON THE 



6. On the Mechanics of Glaciers, with especial Reference to their 

 supposed Power of Excavation. By Eev. A. Irving, B.A., 

 B.Sc, F.G.S. (Bead December 6, 1882.) 



Opinions as to the kind and extent of work done by glaciers upon 

 the rocks over which they move seem still to be very divided in the 

 geological world. On the one hand it is still maintained that 

 these agents are capable of excavating basin-like hollows, such as 

 those which are now (or have been) filled with lakes ; on the other 

 hand we find such high authorities as Profs. Bonney * and Credner f 

 rejecting the hypothesis of excavation, while they fully recognize 

 (as every Alpine observer must do) the scouring, grooving, striating 

 and polishing work done by glaciers upon the floors and sides of 

 valleys previously formed by ordinary valley-erosion, as well as their 

 indirect action in contributing to the formation of lakes by the dams 

 which their moraines form (e. g. at the southern end of Lake Garda) 

 in some cases across valleys. Amid this diversity of opinion I may 

 be pardoned for attempting to add something to the discussion of 

 this interesting subject. 



The whole discussion would seem to narrow itself, theoretically, 

 to the answer to be given to the question, Can a glacier dig or ex- 

 cavate basin-like hollows? 



Those who answer in the affirmative seem in their arguments to 

 assume that the ice of the glacier moves as a rigid mass. If it did 

 so, its scooping-out power would be enormous ; but that it does not 

 has been shown by Prof. Tyndall, in his little work ' Forms of 

 Water ' and elsewhere, and demonstrated experimentally by himself 

 and Prof. Helmholtz J of Berlin. The writings also of Forbes and 

 others on this subject are no doubt familiar to geologists. 



The snow of the upper neve becomes gradually transformed into 

 the solid ice of the glacier in two ways : — (1) By pressure from above 

 the crystalline particles are partly melted, the liquefied portions 

 finding their way between those which still remain solid ; (2) the 

 heat of the sun melts the surface-particles, the water at 0° C. thus 

 formed trickling into the snow. In both cases the water is again 

 transformed into ice, its latent heat being taken up by the snow, 

 which at these high altitudes is at temperatures below 0° C. Rege- 

 lation occurs, as it does behind the shearing-wire in a well-known 

 experiment. Liquefaction by pressure of portions of the ice-mass, 

 and regelation at points where the pressure is relieved, are not con- 

 fined to the neve ; it goes on continuously throughout the mass of 

 the glacier, though more in some parts than in others, and goes a 

 long way to account for the " plasticity " of the ice-mass — its power, 

 that is to say, of adapting itself to the form of the trough or hollow 

 in which it lies. 



* See Quart. Journ. Geol. Soc. vol. xxx. p. 479. 



t ' Elemente der Geologie,' p. 245. \ Vide Lecture, ' Eis und Gletscher.' 



