MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS 71 



The igneous rocks may be divided into various classes according 

 to the percentage of dark constituents present. Any number of 

 divisions might, of course, be made; Shand 1 proposed twelve, 

 though more for descriptive purposes than classificatory. It is, 

 however, not desirable in a classification to multiply excessively the 

 number of classes into which the rocks are divided, and they may 

 be gathered into rather large groups. Tentatively four classes 

 have been made: (1) rocks with less than 5 per cent of dark 

 constituents, (2) dark constituents 

 between 5 and 50 per cent, (3) dark 

 constituents between 50 and 95 per 

 cent, and (4) dark constituents 

 more than 95 per cent. Now since 

 these division lines represent planes 

 parallel to the two quarf eloid planes 

 (quartz-feldspars and feldspars- 

 f eldspathoids) , Fig. 9, they form 

 similar triangles whose sizes repre- 

 sent the amounts of light con- 

 stituents, decreasing with increase v „ c ,,. . . t ., 



. Fig. 10. — subdivisions of the see- 



in dark constituents and approach ondary double tetrahedron into orders. 



to the mafite corner. For conven- 

 ience, however, since they are similar they may be represented by 

 triangles of the same size. 



Thus far the classification is one of five dimensions. But this 

 is not enough. The kind of plagioclase in the rock must be taken 

 into consideration. To bring this factor into the classification, 

 imagine the lozenge-shaped quarfeloid plane to consist of two 

 sheets of paper fastened together only along the Qu-Kf-Foids edge. 

 If now the loose corners of the two sheets be separated a distance 

 equal to a side of the original triangle, a new double tetrahedron 

 will be developed, the horizontal line along which it was opened 

 representing all plagioclases, the ends being formed by the Ab and 

 the An molecules (Fig. 10). The same thing can be done, of course, 

 with the double triangles representing the other classes, and the 

 classification will now be made up of four double tetrahedrons, 



1 S. J. Shand, op. tit., p. 404. 



