SOIL-FORMING ROCKS 185 



and silica, or clay and potassium carbonate. Mica slowly 

 changes to clay and carbonates of the alkalies. Hornblende 

 forms hydrated oxides of iron and aluminium, clay, and 

 carbonates of calcium and magnesium. Quartz changes 

 but little except as to size of particles, these becoming 

 small grains which form sand. 



If the decomposed material is not disturbed it forms a soil 

 of excellent texture and good composition. Apatite occurs 

 very commonly disseminated in small crystals throughout 

 granite, and supplies phosphorus to such a soil. All the 

 other essential elements are present. Sorted by water, 

 however, it separates into sand and clay soils with most of 

 the plant foods in the clay. 



(6) Basalts and Lavas form soils- which may be very 

 sandy and barren, when silica predominates with only cal- 

 cium, magnesium, and aluminium for bases; or form good 

 soils, when bases predominate, which include potassium. 



(c) Limestone Rocks consist for the most part of calcium 

 carbonate (and magnesium carbonate) which is leached out 

 verv completelv bv the action of water and carbon dioxide 

 (Fig. 40). There is left to form soil only a very small per- 

 centage of impurities in the form of clay. Limestone is 

 laid down far out at sea in deep water, and there is entangled 

 with it the finer and lighter particles of clay which have 

 been washed out into the sea bv rivers. This clayey im- 

 purity which forms soil from limestone may have been 

 derived from granite, in which case it would be fairly rich 

 in plant food. It may have been derived from barren 

 basalts, in which case it would be very poor. As a rule, 

 however, limestone soils are rich, except that they may be 

 lacking in calcium carbonate. At first thought it seems 

 strange that limestone soils are apt to be lacking in calcium 

 carbonate. But soil is derived from limestone onlv bv the 

 solution and removal of calcium carbonate, soil material 

 being only the impurities present in the limestone rock. 

 Calcium carbonate is on the whole very soluble in soil water, 

 compared to other rock constituents. Moreover, the particles 

 composing the clay are for the most part decomposition 

 products of silicates which may have originally contained 



