6 SOILS AND, FERTILIZERS 



Gneiss, marble and slate are among the rocks so formed. 

 Gneiss somewhat resembles granite, from which it is 

 formed, but unlike granite has a layered structure, the 

 result of the pressure to which it was subjected. Marble 

 has been formed from limestone or dolomite by heat and 

 pressure, which have caused crystallization. It is not, 

 therefore, so readily soluble as limestone. Slate has been 

 formed from shale by heat and pressure. 



7. Rock-forming minerals. — Most rocks are not homo- 

 geneous, but are made up of a number of different materials. 

 An examination will frequently show grains of different 

 sizes, colors and hardness. The grains are minerals and 

 they differ from each other in their composition as they 

 do in their appearance. But each mineral always has a 

 more or less well-defined composition, so that when we have 

 a certain mineral we know something of the quantity of 

 potash or lime or other base that it contains. The quan- 

 tity of potash or other plant-food material in a rock will 

 depend on the proportion of minerals containing those sub- 

 stances that compose the rock. 



8. Important minerals. — There are a few minerals that 

 it will be well to mention : (1) because they or their products 

 occur in very large quantity in soil and influence its physical 

 properties; (2) because of the plant-food material that 

 they contain. Quartz and feldspar are examples of the class 

 first mentioned. Quartz is found in almost all soils, and 

 may form from 85 to 99 per cent of their composition. It 

 is particularly prevalent in sandy soils. It usually occurs 

 as a large grain, called sand, is hard and insoluble and con- 

 tributes no plant-food material. A soil with a great deal 

 of quartz is usually a light, easily worked soil. 



On weathering feldspar contributes to soils a mass of 

 very finely divided matter known as clay, the smallest of 

 the soil particles. It, therefore, forms part of the clay in 



