70 ALBERT JOHANNSEN 



(3) all plagioclases and the albite molecule in anorthoclase; (4) all 

 f eldspathoids ; (5) the mantes, 1 including the ferromagnesian con- 

 stituents, the "ores," etc., as given below. 



As shown in Fig. 9, the double tetrahedron is unsymmetrically 

 divided on certain faces by the traces of planes parallel to the 

 quarfeloid 2 faces; on others, by lines parallel to one side as well as 

 by lines converging to one of the angles. Experiments were made 

 with symmetrical divisions of various kinds, but it was found 

 impossible to fit the rocks as now named into compartments so 

 made. It is true that new names might have been devised for such 

 subdivisions, but it was not thought desirable to discard entirely 

 the old and well-tried classifications which have very much to recom- 

 mend them besides the fact that they have been so long in use. The 

 old classifications are unsymmetrical, for we speak of a rock as a 

 quartz-syenite, quartz-monzonite, quartz-diorite, etc., when it 

 contains any amount of quartz. With respect to this mineral, there- 

 fore, the classification is based upon its ratio to the sum of all the. 

 other constituents, and the lines of division must be parallel to a 

 side of the tetrahedron. The same is true also of the f eldspathoids. 

 In the divisions according to the feldspars, however, we find, for 

 example, that a rock is a quartz-monzonite whether the percentage 

 of feldspar among the light constituents is 10 or 90. Here the 

 divisions are based upon the ratio of the feldspars to each other, 

 irrespective of what their amount may be in the rock. The division 

 lines, therefore, must converge toward the quartz and feldspathoid 

 corners, as shown in Fig. 9. 



1 When the writer proposed (Jour. Geol., XIX [1911], 319) the term "femag" as a 

 substitute for ferromagnesian minerals which are not minerals of the norm, he did not 

 stop to consider its euphony or whether it fitted into the C.I.P.W. terminology, but 

 thought of it only as a term to take the place of "femic," which was being misused. 

 He is perfectly willing to substitute " mafic" as an adjective, as proposed by the authors 

 of the C.I.P.W. system {Jour. Geol., XX [191 2], 561). He wishes to use here a term for 

 all the dark minerals of a rock except those that are pneumatolytic, and therefore uses 

 "mafite" as a noun, feeling at liberty to include in it, since the word has not been 

 used before, certain iron minerals, as listed below. 



3 C.I.P.W. suggest "felsic" as an adjective for the minerals quartz, feldspars, and 

 f eldspathoids. The writer here uses " quarf eloids " (QUARtz, FELdspar, feld- 

 spathOIDS) as a noun for these minerals in the front faces of the double tetrahedron, 

 "felsite" being unavailable from its use as a rock name. "Leucocrates" cannot be 

 used, since all light-colored minerals are not included. 



