386 DESTRUCTIVE AGENCIES UPON MOUNTAINS. 



point out, are in this respect " continuous, nor have 

 we reason to suppose that they differ materially from 

 age to age." * Then he goes on to pass in review 

 four of the principal agencies which are thus at work, 

 namely (i) Ice wedges. (2) Snow and glaciers. (3) Rain, 

 and (4) Rivers. We shall however at present content 

 ourselves with merely glancing at the operation of the 

 first of these the ice wedges. Water lodges in the 

 joints of the rocks and there freezes. 



" In freezing it expands and tends to push the walls of the 

 joints apart. If by chance a thaw should come, followed by 

 a return of the frost, a further expansion would take place. 

 This process repeated many times goes on season after season : 

 until at last the cohesion of the mass is destroyed, and down 

 goes a thundering heap of ruin to the base of the cliff. 

 Thousands of tons of rock are thus dislodged from the sides 

 of the higher mountains, and sent to strew the slopes all along 

 the bottoms of the valleys." f 



The reader will thus see the enormous influence 

 of that one single agency in reducing the grandest 

 mountain peaks to a heap of ruins; the stupendous 

 power of expansion exercised by water in turning 

 to ice is something almost incredible. In practice there 

 is nothing that can resist it. Water, as we know, 

 attains its greatest density at 39.2 Fahr., after which 

 it slowly expands, until a temperature of 32 F. is 

 reached, at which point congelation takes place; 

 and in freezing " it suddenly expands about one- 

 eleventh of its bulk, with almost irresistible power; and 

 a pressure as high as 28,000 Ibs. to the square inch 



* Mountain Architecture, a Lecture delivered in the City Hall, 



Glasgow, by Professor Sir Archibald Geikie, 27th January 1876. 

 Published 1877, p. 19. 



j- Professor Sir A. Geikie in Ibid., p. 20. 



