74 



ALBERT JOHANNSEN 



making a third division. In the granite and syenite, monzonite 

 and quartz-monzonite, syenodiorite and granodiorite families a 

 line separating 50 per cent light from 50 per cent dark would 

 throw practically all of the rocks on the same side. With respect 

 to diorite and quartz-diorite the curve is not good, owing to 

 insufficient data, and it shows no definite maximum. The gabbro- 

 curve has its maximum at 60 per cent light. With the gabbros 



dark 5 10 



20 30 40 50 60 70 

 Percentage of light minerals 



80 



90 



10 

 5 







5 

 

 & 







5 







6 





 70 

 50 

 30 

 10 

 100°/ light 



Fig. 12. — Curves showing the number of rocks with various percentages of 

 light and dark constituents: A, granite and syenite; B, quartz-monzonite and monzo- 

 nite; C, granodiorite and syenodiorite; D, quartz-diorite and diorite; E, quartz-gabbro 

 and gabbro; F, all rocks. 



and diorites it might be better to make five classes with dividing 

 lines at 0-5-35-65-95-100 instead of at 0-5-50-95-100, yet the 65 

 per cent light line cuts the gabbro-curve at rather a high point. The 

 addition of a fifth class for rocks with approximately equal amounts 

 of light and dark constituents would increase the total number of 

 families by 104, and to the writer it seems undesirable to do this. 

 Not much is gained, and it is just as well to speak of light and dark 

 gabbros, separating on the 50-50 line, as to make the main gabbro 

 class the intermediate 35-65 position. In the lowest curve, which 



