MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS 95 

 Kf 21.5 39.0 



Rock number is 2223. The values 39 and 61 are used in 

 plotting the rock. 



Example 4 — A lherzolite with 



Augite 45. o = 47 . 4 



Hypersthene 20.0 = 21.0 



Olivine 30. o = 31.6 



Hornblende 3.0 100 . o 



Magnetite 2.0 



100. o 



Since there are neither feldspars, feldspathoids, nor quartz, the 

 rock must belong in Class 4. 



The ratio of ferromagnesian minerals to ores is 98 : 2, therefore 

 the Order is 1 . 



The ratio of augite to hypersthene is 45 : 20= 69 : 31, therefore the 

 family lies in the middle row and is either 1, 3, 9, or 15 (Fig. 19). 

 The ratio of augite to olivine is 45:30=60:40, and the rock again 

 lies in the middle line including Families 2, 3, 10, and 15. The 

 ratio of augite to hornblende is 45:3 = 95:6, therefore it is in 

 the front series of families including 1, 2, 3, 4, 8, 9, 10. Family 3 is 

 the only one common to the three computations, consequently the 

 rock number is 413. 



Graphically the rock may be plotted by using the numbers 47 . 4, 

 21.0, and 31.6. 



One of the advantages of this system of classification is that 

 each thin section of the rock may be plotted independently; the 

 center point of all the dots representing sections from a single rock- 

 mass will represent the average. This is much more satisfactory 

 than estimating the average from a number of sections which differ 

 considerably in the amounts of the constituents. The various 

 dots representing complementary rocks will fall in straight or 

 branching lines, showing the course of differentiation. 



Before publishing his second paper on this system of classi- 

 fication the writer desires the opinions of more petrographers than 

 he has been able to consult personally. He would be very glad, 



