ORIGIN AND FOEMATION OF SOILS. 19 



by the action of water. The igneous rocks form but a 

 small proportion of the outer rocks of the world; yet 

 they are of great importance, because it is through their 

 decay or breaking up that the other rocks have been 

 formed. 



The most important of the igneous rocks are granite 

 and trap. These contain silica and alumina as their chief 

 constituents, and greater or less amounts of potash, lime, 

 magnesia, iron, and the other mineral constituents of 

 plants. These rocks are also called " primary rocks," 

 because they are supposed to constitute the primary or 

 first crust of the earth, and to be similar in character 

 and composition to the now molten interior of the earth. 

 This view is based upon the fact that the lava ejected 

 from active volcanoes resembles in composition the trap 

 rocks. 



The trap rocks consist mainly of two minerals, feldspar 

 and hornblende. Feldspar is particularly rich in potash 

 and soda, and poor in lime and magnesia. Hornblende 

 is poor in potash and soda, and rich in lime and mag- 

 nesia. Granite rocks consist mainly of quartz, feldspar, 

 and mica. Quartz is almost pure silica; while mica con- 

 tains nearly all the compounds found in both feldspar and 

 hornblende, and is one of the most abundant minerals. 



The aqueous rocks are formed from the minerals con- 

 tained in igneous rocks, the proportion of the different 

 constituents contained in them depending upon the method 

 of their formation. Limestone and red sandstone are 

 good examples of aqueous rocks. 



Subsoil, — The subsoil lies immediately underneath 

 the soil, and rests upon the solid rock. The main dis- 



