SOIL MATERIAL AND ITS ORIGIN 7 



(a) Granite-Rhyolite. This group is composed of quartz, 

 feldspars, chiefly orthoclase and albite, mica, either hlack or white, 

 and ampliibole or pyroxene. A small amount of apatite is always 

 present. Rhyolite is the principal volcanic rock of this group. In 

 the decomposition of this group carbonated water attacks the feld- 

 spars, dissolving out the potassium and sodium in the form of car- 

 bonates, leaving the clay residue. The quartz is broken down into 

 sand and gravel while the other minerals are decomposed into other 

 products as given under those special minerals above. The resulting 

 soil material formed is a sandy clay, the color depending upon the 

 amount of iron-bearing minerals in the granite. The soil material 

 formed from rhyolites and other volcanic granites differs only in 

 fineness from that resulting from intrusive granites. 



(b) Syenite-Trachyte. This group consists chiefly of the 

 feldspars, orthoclase or albite, minerals of the mica, amphibole or 

 pyroxene groups and a small amount of apatite. It ditfcrs_from the 

 granites jn the absence of quartz. Its decomposition is simJlar^o 

 that of the granite but gives a clay free from sand, colored by iron 

 compounds. This rock is not so common as granites. Trachyte is 

 the volcanic form of syenite. 



(c) Diorite-Andesite. These rocks contain oligoclase, mica, 

 usually biotite, amphibole or pyroxene and apatite. Quartz may 

 be present. The soil material formed is cither a rather highly 

 colored clay or sanely clay, depending on the absence or presence of 

 quartz in the original rock. Andesite is a common volcanic form. 



(d) Diabase-Basalt. This group of rocks consists of labrado- 

 rite, amphibole or pyroxene, usually the latter, and small amounts of 

 apatite. Quartz may be present in small quantities as in the case of 

 the diorites. Usually large amounts of magnesium and iron-bearing 

 minerals are present. The decomposition of this group gives a 

 highly colored clay containing large amounts of hydrated mag- 

 nesium silicates. Basalt is the volcanic form. 



Other groups of igneous rocks might be given, but these are 

 sufficient to illustrate the changes that take place in the formation 

 of soil material from them. 



2. Aqueous rocks are divided into three classes, (a) those 

 whose constituents have been in solution and have been deposited 

 by cooling, evaporation, release of pressure or by direct chemical 

 precipitation; (b), sedimental or fragrnental deposits, those formed 

 by the breaking down of preexisting rocks and deposited by the 

 action of water; and (c), those formed largely by plants and 



