42 TOWN GEOLOGY. [i. 



If you go to the Alps at certain seasons, and hear the 

 thunder of the falling rocks, and see their long lines 

 moraines, as they are called sliding slowly down upon 

 the surface of the glacier, then you will be ready to 

 believe the geologist who tells you that frost, and 

 probably frost alone, has hewn out such a peak as the 

 Matterhorn from some vast tableland ; and is hewing 

 it down still, winter after winter, till some day, where 

 the snow Alps now stand, there shall be rolling uplands 

 of rich cultivable soil. 



So much for the mechanical action of rain, in the 

 shape of ice. Now a few words on its chemical action. 



Rain water is seldom pure. It carries in it carbonic 

 acid ; and that acid, beating in shower after shower 

 against the face of a cliff especially if it be a lime- 

 stone cliff weathers the rock chemically; changing 

 (in case of limestone) the insoluble carbonate of lime 

 into a soluble bicarbonate, and carrying that away in 

 water, which, however clear, is still hard. Hard water 

 is usually water which has invisible lime in it ; there 

 are from ten to fifteen grains and more of lime in 

 every gallon of limestone water. I leave you to 

 calculate the enormous weight of lime which must be 

 so carried down to the sea every year by a single lime- 

 stone or chalk brook. You can calculate it, if you 

 like, by ascertaining the weight of lime in each gallon, 

 and the average quantity of water which comes down 

 the stream in a day ; and when your sum is done, you 

 will be astonished to find it one not of many pounds, 

 but probably of many tons, of solid lime, which you 

 never suspected or missed from the hills around. 

 Again, by the time the rain has sunk through the soil, 

 it is still less pure. It carries with it not only carbonic 



