CLA SSIFICA TION OF IGNEO US ROCKS 5 7 1 



distinguished by chemical characters, but also by associations in 

 the rocks. They are : 



a) Silica and alumina uncombined, quartz (tridymite) and 

 corundum, together with zircon, which, though commonly present 

 in very small amount, is.oftenest found in rocks rich in silica or 

 alumina. 



b) Aluminous ?wn-ferromagnesian minerals ; orthoclase, albite, 

 anorthite and mixtures of these, leucite, analcite, nephelite, soda- 

 lite, hauynite, noselite, cancrinite, and muscovite. 



c) Aluminous ferromagiiesian and calcic silicates (interme- 

 diate between b) and d) ) : aluminous pyroxenes and amphiboles, 

 biotite, garnet, tourmaline, melilite, some spinels, etc. 



d) N o?i- aluminous ferromagnesian and calcic silicates: hyper- 

 sthene (including enstatite) , diopside (including hedenbergite), 

 acmite, olivine (including fayalite and forsterite), and aker- 

 manite. 



e) Non-siliceous and non-aluminous minerals with titanosilicates; 

 magnetite, hematite, ilmenite, apatite, titanite, perofskite, and 

 fluorite, together with the native metals, and certain other 

 metallic oxides and sulphides. 



If igneous rocks are considered from the standpoint of their 

 mineral composition, they are found to consist of graduating 

 series of quantitatively different mixtures of several groups of 

 minerals, and since they are all necessary to an exact expression 

 of the chemical composition of the rock, each mineral or group 

 of minerals should receive proper recognition according to its 

 quantitative value. Owing to the number of minerals in most 

 rocks, this is a very intricate problem and we have made repeated 

 attempts to solve it by recognizing several independent mineral 

 factors at one time, involving the problem of handling three or 

 more co-ordinate quantities. This was found impracticable as a 

 basis of classification, and it was seen that the most feasible pro- 

 cedure is to recognize such factors successively according to 

 certain degress of qualities or magnitudes possessed by them in 

 comparison with one another. This has been done by grouping 

 them on a basis of chemical identity or resemblance, and of 



