MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS 69 



and hypabyssal rocks must be regarded as modifications of these. 

 In this paper the writer has given names only to the plutonic 

 representatives of the few families considered, it being understood, 

 of course, that the granite family includes rhyolites; the syenite 

 family, trachyte; the monzonite, latite; etc. 



The basis ofrthe classification here proposed is a double tetra- 

 hedron (Fig. 9), each trihedral angle of which represents certain 

 mineral constituents. If there 



were a geometrical figure hav- J$\--cia6s 1 



ing ten or twelve corners, each y/fll\>> Class 2 



equally distant from each of the 



others, it would have been pos- //// nV^^L- A. class 4 



sible to use a single mineral at Kf ^^^JjJ.j^ :r: —JS^^^ V i eL fii eB 

 a corner. Since there is no such 

 figure, and rocks must be lo- 

 cated with reference to all of 

 the minerals which occur in 

 them, it was found necessary Folds 



to divide the minerals into as ^ „,,... 



Fig. 9. — Subdivisions 01 the double 



many groups as there are cor- tetrahedron into classes. 

 ners in a tetrahedron. But 



quartz and the feldspathoids never occur together, so it was possible 

 to make the classification in five dimensions by using two tetrahe- 

 drons with a common base (Fig. 9) . This arrangement was found 

 to answer the purpose admirably, for the relationships between 

 rocks which may contain either quartz or nephelite, etc., and which 

 appear anomalous in the old classifications, are clearly shown. 



The groups of minerals represented by the corners of the double 

 tetrahedron are: (1) quartz (symbol Qu 1 ); (2) potash feldspars 

 (symbol Kf), including the orthoclase molecule in anorthoclase; 



1 In the figures in this paper the quartz corner is indicated by the symbol Qu. 

 The 'letter "f " is used for feldspar, therefore Kf indicates the potash-feldspars — 

 orthoclase, microcline, and the orthoclase molecule in anorthoclase; Naf indicates 

 albite and the soda molecule in anorthoclase, while CaiVaf represents the acid plagio- 

 clases and NaC<zf the basic plagioclases, the element in excess being given in italics 

 temporarily to avoid confusion, although there need be none if one thinks of the symbol 

 as reading calcium-bearing soda feldspar for the acid plagioclase and soda-bearing 

 calcium plagioclase for the basic. Caf is used for anorthite, and Foids for the feld- 

 spathoids, lenads being unavailable from its use for certain normative minerals of the 

 C.I.P.W. system. 



