186 GLACIAL HISTORY. 



that the advance of the glacier takes place during the whole 

 year, although it is much greater in the spring and in the be- 

 ginning of summer than during the winter, but that the volume 

 of the ice and the inclination of the ground beneath it exert a 

 great influence upon the progressive movement ; and thus each 

 glacier has a peculiar rate of progress. The Mer-de- Glace of 

 Mont Blanc from 1788 to 1832 advanced* at the annual rate, 

 on the average, of 114 metres (or nearly 125 yards) ; in the 

 Unteraar glacier, according to Agassiz, the annual mean advance 

 from 1841 to 1846 oscillated between 52 and 71 metres (or from 

 nearly 57 to 77*648 yards) ; in the upper part of the glacier of 

 the Aar an advance of 1428 metres (or about 156'1 yards) took 

 place from 1827 to 1840, being at the annual rate of about 

 100 metres (or 109 yards). 



Rigid and compact as the ice of a glacier appears, it is never- 

 theless very slowly and continously moving downwards, and it 

 may be compared to a stream which, in obedience to the laws of 

 gravity, flows slowly towards the low ground. In this way the 

 masses of snow which are produced during the greater part of 

 the year upon the remote deserts of the granular snow-fields, 

 gradually descend into warmer regions, where they melt, and 

 with them are carried along any foreign bodies that may have 

 been deposited upon the back of the glacier. The Swiss moun- 

 tains are exposed to a slow but continuous wearing and destruc- 

 tion by atmospheric influences. Even from the most solid rocks 

 certain materials are slowly dissolved by the action of air and 

 water ; small pores and fissures are formed, into which water 

 penetrates, and this by its irresistible expansion, during the 

 constantly recurring frosts, splits the rock. Violent storms and 

 torrents of rain set in motion the loosened fragments, which 

 rush down carrying other fragments with them. In the region 

 of the granular snow-fields there is less of disintegration, since 

 an unbroken mantle of snow covers the entire surface of the 



* [The advance of the glacier is thus described by Lord Byron in his dra- 

 matic poem of Manfred : 



" The glacier's cold and restless mass 

 Moves onward day by day." 



EDITOR.] 



