WEATHERING OF ROCKS 75 



The result of the decomposition of granite, then, is the forma- 

 tion of a mass of clay, through which are disseminated the un- 

 changed grains of quartz and mica. In the other igneous rocks 

 the mode of disintegration is essentially the same : the complex 

 silicates are decomposed into simpler compounds, clay being the 

 principal derivative of the felspars, while the quartz, if present, is 

 broken up into fragments and forms sand. Even when an igneous 

 rock is yet firm and hard and, examined by the naked eye, appears 

 to be quite unchanged, the microscope often reveals important 

 chemical changes, which are the first steps of decay. 



The circulation of the material of rocks is continuous, and rocks 

 which are themselves composed of substances derived from the 

 decay of older rocks are attacked in their turn and yield material 

 for new formations. These derivative rocks, such as sandstones, 

 slates, and limestones, are affected in characteristic ways by the 

 rain. 



Sandstones are composed of grains of sand (quartz, Si0 2 ) ce- 

 mented together ; the cementing substance may be silica itself, 

 some compound of iron, such as Fe 2 3 , or carbonate of lime 

 (CaC0 3 ), and the dissolving away of the cement causes the rock 

 to crumble into sand. In a sandstone with siliceous cement the 

 action is excessively slow, atmospheric waters having very little 

 effect upon silica, but underground the humous acids are believed to 

 dissolve it slightly. Ferric oxide (Fe 2 3 ) is likewise unchanged 

 by rain-water, but beneath the soil decomposing organic substances 

 deoxidize it into FeO, which, taking up C0 2 , forms the soluble 

 carbonate of iron (FeC0 3 ). The uppermost layers of red sand- 

 stone are often thus completely disintegrated into loose sand, 

 bleached by the removal of the iron which gave it its colour. 

 Carbonate of lime is very soluble in water containing carbon diox- 

 ide, as all rain-water does, and in sandstones with calcareous 

 cement, disintegration is rapid. In sandstones and slates it is the 

 cementing substance which is removed, leaving the grains of sand 

 or particles of clay unchanged, and the limestones are simply dis- 

 solved. This is because the materials of these rocks were, for the 

 most part, originally derived from the decomposition of the igneous 



