ROCK CLASSIFICATION 95 



upon a chemical basis in nearly every instance is the grouping 

 together of rocks that have like proportions of the same chem- 

 ical elements. 



The mineral components of igneous rocks are somewhat less 

 constant in character than the chemical elements. Rocks occur 

 that have distinctly different mineral components such as a 

 granite with quartz, orthoclase and biotite, and a gabbro with 

 labradorite, augite and hypersthene. But others occur having 

 the same kinds of minerals in quite different proportions, such 

 as a granite with much quartz and orthoclase and little biotite, 

 and a syenite with much orthoclase and little quartz and biotite. 

 Neglecting for the present the rarer or least abundant minerals 

 that are found in igneous rocks, the characteristic ones are : 

 quartz, feldspar, leucite, nephelite, sodalites, analcite, micas, 

 pyroxenes, amphiboles, olivine, magnetite. Of these only one 

 possesses a fixed composition, that is, quartz. Magnetite may 

 contain a variable amount of titanium. Leucite, nephelite, and 

 analcite may vary in the relative proportions of potassium and 

 sodium present. Olivines differ in the proportions of iron and 

 magnesium. While the others represent series of minerals 

 grouped together on crystallographic grounds, but varying often 

 widely in chemical composition. The feldspars embrace poly- 

 silicates with variable amounts of potassium and sodium, besides 

 compounds assumed to consist of polysilicate and orthosilicate, 

 varying in alkali metals, calcium and silicon. Micas, pyroxenes, 

 and amphiboles present groups which are chemically still more 

 variable. The chief mineral components of rocks then are not 

 definite chemical compounds, but are substances that may vary 

 within limits according to the proportions of the chemical ele- 

 ments in the magma from which the rock solidified. Moreover 

 the same chemical elements appear in several of these minerals : 

 oxygen in all, silicon in all but one, aluminium and the alkali 

 metals in five, iron and magnesium in four, calcium in three and 

 so on. Hence a change in the relative proportions of the 

 chemical elements in a magma may affect nearly all of the min- 

 eral constituents. 



