y6 THE ATMOSPHERE — RAIN 



rocks, and the minerals which compose them are already of a 

 very simple and stable character. 



The sandstones are largely employed for building materials, and 

 their value and permanence for such purposes depend principally 

 upon the character of the cementing substances in them. For 

 this reason, the siliceous and ferruginous sandstones are the most 

 durable, those with calcareous cements usually yielding with com- 

 parative rapidity to the attacks of the weather. 



Slates and shales, by removal of their soluble constituents, 

 crumble down into clay. 



Limestones are among the few rocks which are chiefly or entirely 

 made up of soluble material, the carbonate of lime (CaC0 3 ). 

 This is attacked by the rain-water, dissolved and carried away in 

 solution, while the insoluble impurities contained in the rock 

 remain to form soil. The proportion of such impurities varies 

 greatly in different limestones, and hence the residual soil will 

 vary, but it is generally a clay, since that is much the commonest 

 of the impurities in limestone. Sand also occurs in limestones, 

 either with or without clay. When the sand forms a coherent 

 mass, out of which the calcareous material has been dissolved, it 

 is called rotten-stone. 



The gradual formation of soil by the disintegration of rock may 

 be easily observed in excavations, even shallow ones, such as cel- 

 lars, wells, railroad cuttings, and the like. At the surface is the 

 true soil, which is usually dark coloured, due partly to the admixt- 

 ure of vegetable mould, partly to the complete oxidation and hy- 

 dration of its minerals. Next follows the subsoil, which, owing to 

 the absence of vegetable matter and the less complete oxidation 

 and hydration, is of a lighter colour. The subsoil is frequently 

 divided into distinct layers, and often contains unaltered masses 

 of the parent rock, which have resisted decomposition, while the 

 surrounding parts have become entirely disintegrated. By im- 

 perceptible gradations the subsoil shades into what looks like 

 unaltered rock, but is friable and crumbles in the fingers ; this is 

 rotten rock. From this to the firm, unchanged rock, the passage 

 is equally gradual. 



