SOME QMNEBAL COJ^SIDEBATIOJSrS 7 



The igneous rocks furnish material for the formation 

 of the types constituting the other groups. They may 

 be divided in a general way into two classes — one con- 

 taining a high percentage of silica and some free quartz, 

 the other having a medium or low silica content and no 

 quartz. The former is designated as acid, and the latter 

 as basic since it contains a high percentage of the alkalies 

 and the alkaline earth minerals. Granite and gabbro 

 are excellent examples, respectively, of these general 

 groups of rock. 



The sedimentary rocks, formed from material derived 

 from the igneous rocks, have been deposited usually 

 under fresh- or salt-water conditions. The development 

 of pressure has in many cases been instrumental in the 

 consolidation of this material. The limestone and the 

 dolomite deposited by precipitation may be expected 

 to be comparatively soluble rocks. Shale is merely a 

 more or less hardened clay, while sandstone varies ac- 

 cording to the cementing material which serves to hold 

 its sand grains together. This cement may be iron 

 (Fe203), calcium carbonate (CaCOs), or silica (Si02). 



The action of heat, usually with pressure, on either 

 igneous or sedimentary rocks, results in the third group, 

 the metamorphic. Thus, granite on metamorphosis 

 may form either a gneiss or a schist ; limestone or dolomite 

 may form marble ; shale may form slate ; and sandstone 

 may form quartzite. 



On examination, ordinary rock is found to be composed 

 of one or more minerals. In other words, it is a mineral 

 aggregate. The mineral, in turn, is a natxu-al compound 

 of approximately a constant chemical composition, usually 

 displaying a crystalline form and other well-defined 

 physical properties. In order to illustrate the com- 



