MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS 79 



The vertical direction of Fig. 16 gives the quartz percentage. In 

 Fig. 15 are plotted the curves for the proportion of quartz among 

 the light constituents for all rocks in the upper triangle (Families 

 o to 15, Fig. 16), and separate curves for Orders 1, 2, and 3. The 

 separation at 5 is clear. There may be a question whether the 

 upper division of quartz should be made at 95, 90, or even at 65. 

 For symmetry, of course, it should be at 95. With respect to a line 

 at 50, the writer is in doubt. Practically all the rocks fall below 

 50 per cent quartz (that is, quartz is less than 50 per cent of the 



100 90 



80 



Percentage of quartz 

 70 60 50 40 30 



20 



10 



|0 



60 



60 



40 



20 



Fig. 15. — Percentage of quartz among the light constituents, recalculated to 100. 

 Curves for Orders 1, 2, 3, 4, and totals. The numbers indicate the orders. 



light constituents, consequently it forms even less than 50 per 

 cent of all the constituents of the rock). It would be possible to 

 group all the rocks given in Families 2 and 7, 3 and 8, 4 and 9, etc. 

 (Fig. 16), together, and call those falling in the upper divisions 

 simply quartz-rich granites, etc. However, since there are so few 

 rocks here, it may make it all the more desirable to divide on the 

 50-50 line. This would make uniform divisions everywhere 

 in the system at 0-5-50-95-100 except for the Kf-Plag ratio. Of 

 course the retention of the line at 50 in this and the lower triangle 

 makes 8 or 10 more families in each order of the first three classes, 

 or a total of 102. However, if these families are simply numbered 

 and the rocks called quartz-rich granite, quartz-rich granodiorite, 

 nephelite-rich nephelite-syenite, etc., it will add no new names and 

 make clearer the positions of the rocks. Curves drawn for the 



