154 PHYSIOGRAPHY. [chap. 



has seen " pieces of rock, eight pounds in weight, raised by a 

 mass from the bottom and carried down the river." ' Here 

 then is a geological agent not to be neglected, since it 

 assists the transporting power of stfeams in carrying solid 

 matter from the land seawards. But, if the geological im- 

 portance of ice is to be fully Realized, attention must be 

 turned from these trivial illustrations to the grand spectacles 

 which are presented by masses of moving ice in mountain 

 regions beyond the limits of our own islands. 



When a snow-storm occurs in' this country, the snow does 

 little or nothing in the way of denudation, beyond what it 

 may effect indirectly, by giving rise to floods when a rapid 

 thaw takes place. In fact, the snow, as snow, protects 

 rather than destroys. Bat the result is different in a 

 mountainous countr)^, such' as that of the Swiss Alps. The 

 greater parf of the snow whichfalls there above the snow-line, 

 as explained in Chapter IV., lies all the year round un- 

 melted ] and, therefore, every fall must needs add to the 

 thickness of the heap piled upon the moUntain-top. It is 

 true that the snow evaporates, but the evaporation is 

 extremely slow,- and is far from- equal to the additions con- 

 stantly received ; and, though the heat of the sun during 

 the day, may melt the surface Ikyer, the water thus formed 

 sinks in and becomes frozen in the interior of the mass. 

 Occasionally, the accumulation is relieved by a great mass 

 of snow sliding down the mountain slope, as an avalanche. 

 Usually, however, the pressure of the heaped-up snow gets 

 rid of the surplus by gently squeezing it into the valleys 

 below, where it moves down with extreme slowness. Yet 

 it does not come down as a mass of white opaque snow. 

 It has been shown in art earlier part of this work (p. 64) 

 that snow is white and opaque in consequence of the 

 1 Report of the Tha7iies Commissioners, Appendix i. 1866. 



