1922] Hudson: Geology of the Cuyamaca Region of California 223 



of the percentage of each mineral by an appropriate factor, the factors 

 being chosen to bring the percentage of each mineral to the same pro- 

 portion with respect to (MgO, FeO)Si0 2 . The factors are: olivine, 

 1.4; lrypersthene, 1.0; augite, 0.5; green hornblende, 0.5; brown horn- 

 blende, 0.66. The curves for the labradorite-bytownite and bytownite 

 rocks show a direct proportion between the amount of sulphide and 

 the sum of the amounts of ferromagnesian minerals. The curve for 



I 



Eg 



I 

 ^7 



° rt'" x .X. 



„ . Character izind 

 Graphs feldspars 



■ O Afid-Lob^Andes Hie, 



Ot'goc/ass. 



— X Labradorite. 



- O L abtadoritt - ByicnmTs 



—^B/tonnitc 



IC SO 40 SO 60 TO SO 30 



Sum of ferrtmogriesfm mi/ierab^ 



Fig. 5. The four curves represent the relations between the ores and the 

 ferromagnesian silicates for four different groups of the basic intrusive rocks, 

 characterized by four different plagioelase feldspars. The sum of the ferro- 

 magnesian minerals for each rock was obtained by addition of the products of 

 the percentage of each mineral in the rock by an appropriate factor. These 

 factors were used so as to make the various sums proportioned with respect to 

 (MgO, FeO) SiOo. These factors are: olivine, 1.4; hypersthene, 1.0; augite, 0.5; 

 green hornblende, 0.5 ; brown hornblende, 0.66. 



the labradorite rocks is without character and proves nothing. The 

 curve for the soda-rich plagioelase rocks suggests a relationship be- 

 tween the amount of magnetite and that of the ferromagnesian min- 

 erals, but not enough samples of these rocks were examined to make 

 this a good case. 



The recognition of a reciprocal relationship between the amount 

 of pyrrhotite and the amount of magnetite in the basic rocks has led 

 to the idea of plotting the quotient of magnetite by pyrrhotite against 



