xiii Action of Water on the Land 237 



rock which is dissolved most readily. The waste of the 

 hard and massive surface is often shown only by the way 

 in which it becomes studded with less soluble nodules or 

 fossils originally hidden in its substance. Sir Archibald 

 Geikie has calculated that by the acid-laden rain of towns 

 one-third of an inch is removed from the surface of marble 

 monuments in a century. Insoluble sulphides, such as 

 that of iron, are rapidly oxidised by air in the presence of 

 moisture to form soluble sulphates, and when this process 

 goes on in the pores of a rock the expansion of the crystal- 

 lised salt splits the block into thin layers. This action is 

 the basis of the common way of making alum. In the case 

 of granite ( 43) and most other rocks the process of 

 weathering is more complicated. Some of the minerals are 

 decomposed. In felspar, for instance, the silicates of potash, 

 soda, and lime are changed to carbonates which are washed 

 away, while the silica and the more resisting silicate of 

 alumina remain as a soft crust of kaolin or china clay, 

 valuable for making porcelain. Granite has been found 

 weathered in this way in South America to the depth of 600 

 feet. Rocks containing, iron usually become brown or 

 reddish in colour, although the freshly broken rock may be 

 white or gray. The lines of stratification and joints ( 290) 

 of rocks are sometimes etched out by weathering, so that 

 the face of a cliff assumes the appearance of a gigantic wall 

 of masonry. The crumbling of rocks in rainy regions is 

 assisted by the action of the Sun in drying and warming 

 the surface, which may then be splintered into flakes by a 

 shower of cold rain. Rain soaking by capillary attraction 

 ( 39) through the weathered crust and into the pores of 

 the solid rock is frozen in cold weather, and the ice, ex- 

 panding as it forms, acts like a multitude of minute wedges 

 driven simultaneously in all directions. When the thaw 

 comes, the bases of cliffs and banks are strewn with 

 weathered crusts and stones, often of a great size, broken 

 off in this way. 



311. Soil. Weathered rock is the basis of soil, which 

 accumulates to the greatest depth on level or slightly- inclined 

 land. When the rocks yield only angular grains of quartz 



