74 THE ATMOSPHERE — RAIN 



sound and fresh. It was noted, however, that on exposure to the 

 atmosphere, such not infrequently shortly fell away to the condi- 

 tion of sand" (Merrill). 



Nearly all rocks contain materials which are more or less solu- 

 ble or destructible in rain-water, though usually these soluble ma- 

 terials make up but a small proportion of the whole. When they 

 are removed, the rock crumbles into a friable mass which, on 

 complete disintegration, forms soil. Frozen soil, though with only 

 a small quantity of ice in it, is as hard as many rocks, the ice act- 

 ing as a cement and firmly binding the granules together. When 

 the cementing ice loses its coherence by melting, the rock-like 

 mass becomes friable. This illustrates the effect of removing a 

 small quantity of soluble material from a hard rock. Except vege- 

 table moulds, all soils are derived from the disintegration of rocks. 



The materials out of which rocks are made vary so much that 

 the chemical processes which destroy them must be correspond- 

 ingly different. Our survey of the disintegrating agencies will 

 naturally begin with those rocks which have once been melted, 

 or the igneous rocks, because, as we believe, the first solid crust 

 of the globe was formed of such rocks, and from them started the 

 cycles of change and circulation of matter which are still going on. 



The great majority of the igneous rocks are made up of crystals 

 of some variety of felspar (see p. 16) associated with other min- 

 erals, such as hornblende (p. 20), mica (p. 18), quartz (p. 15), etc. 

 As an example, we may take granite, which is composed of ortho- 

 clase felspar, quartz (Si0 2 ), and mica or hornblende, both of which 

 are complex silicates of iron, lime, etc. Rain-water falling upon 

 an exposed mass of granite, or reaching it by underground perco- 

 lation, slowly decomposes the orthoclase, either removing the 

 potash in the form of a soluble silicate, or converting the silicate 

 into carbonate. The silicate of alumina, which is left behind, 

 absorbs water and forms clay or kaolin (A1 2 3 , 2 Si0 2 , 2 H 2 0). 

 The quartz is unaffected by the rain-water, being insoluble and 

 a very simple and stable compound, which is not decomposed 

 into simpler substances by ordinary natural agents. The mica 

 is very slowly disintegrated. 



