M1NERAL0GICAL CLASSIFICATION OF IGNEOUS ROCKS 65 



chemical compositions, but in most cases their chemical composi- 

 tions are as far from true granites as are their mineral compositions. 

 The figure shows that there are actually 6 potash-granites (one of 

 them quartz-rich), 63 normal granites (4 of them quartz-rich), 

 29 quartz-monzonites (1 of them quartz- ri ch) , and 11 granodiorites. 

 Fig. 2 represents 30 so-called "syenites." There are 2 potash- 

 syenites, 3 normal sye- 

 nites, 14 normal granites, 

 3 monzonites, 7 quartz- 

 monzonites, and 1 grano- 

 diorite. 



Many recent papers 

 show the tendency toward 

 a quantitative mineralogi- 

 cal classification. Thus 

 Brogger proposed 

 fairly definite boun- 

 daries for monzonite 

 and quartz-monzo- 

 nite. From the latter 

 Lindgren separated 



granodiorite, and established limits so clearly that almost all rocks 

 described as granodiorites are actually such. But covering a wider 

 field are later papers bylddings 1 and Lincoln. 2 Each of these writers 

 proposed a definite classification, and more recently Shand 3 sug- 

 gested subdividing rocks according to their percentages of light 

 and dark constituents. To the writer, none of these classifica- 

 tions appears so satisfactory as that which he has presented to 

 his students, with various modifications, during the past seven 

 years. The system was first thought out in the summer of 1909, 

 and even so long ago as the summer of 19 10 the writer pre- 

 pared plaster models of tetrahedrons, cut into subdivisions essen- 

 tially as shown here. Owing to press of other work and lack of 



1 Joseph P. Iddings, Igneous Rocks (New York, 1913), Vol. II. 



2 Francis Church Lincoln, "The Quantitative Mineralogical Classification of 

 Gradational Rocks," Econ. Geol., VIII (1913), 551-64. 



3 S. J. Shand, "A Recording Micrometer for Geometrical Rock Analysis," Jour. 

 Geol., XXIV (19 1 6), 404. 



Fig. 2. — Thirty so-called "syenites" 



