THE VARIOUS ROCKS AS SOIL-FORMERS. 



49 



The clay rocks form the most varied products under the in- 

 fluence of (aqueo-igneous) metamorphism ; granites, gneiss, 

 syenite and hornblendic schist are among the most common. 

 The great variations in the composition of clayey materials 

 account for the correspondingly great variations in the nature 

 of the resultant metamorphic rocks. 



Igneous or Eruptive Rocks. These are usually divided into 

 two groups ; the one characterized by a large proportion of free 

 quartz (silicic acid), and hence designated as acidic, and usu- 

 ally of a light tint; the other the basic, containing little or no 

 free quartz, and commonly of a dark tint caused by the pres- 

 ence of a large amount of iron (contained in pyroxene, more 

 rarely in hornblende). 



Of the latter class are the dark " basaltic " rocks constituting the 

 mass of the enormous eruptive sheet of the Pacific Northwest, covering 

 the greater part of Washington, Oregon and northeastern California. 

 The lavas of the Hawaiian islands are of the same class and even more 

 basic ; while the eruptives of Nevada, middle and southern California, 

 and eastward to the Rocky Mountains, are mostly of the light-colored, 

 acidic type. The same is largely true of the rocks of the Andes of 

 Central and South America, the gray " Andesites," also represented in 

 the Caucasus. 



As one and the same eruptive material may, according to 

 the greater or less rapidity of cooling, appear as a glassy mass 

 (obsidian, pumice, volcanic ash, tuff, etc..) or as a crystalline 

 rock resembling coarse granite in structure, it is not easy to 

 identify them in all their various forms. This can frequently 

 be done only by ascertaining their component minerals by the 

 microscope, or by chemical analysis. The same is sometimes 

 true of metamorphic rocks; and as in the latter, the several 

 feldspars and quartz, with pyroxene instead of hornblende, con- 

 stitute the predominant soil-forming minerals. More rarely, 

 garnet, chrysolite, leucite and other silicates require considera- 

 tion. 



Generalities regarding the Soils derived from various Rocks. 



It is hardly necessary to insist that as in the case of the 

 rocks composed of single minerals, already referred to above. 

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