222 University of California Publications in Geology [Vol. 13 



a curve representing the chances of finding pyrrhotite in the gabbros, 

 norites, die-rites, etc., that are characterized by the different feldspars 

 of the soda-lime series. It is plotted from the results of study of 82 

 specimens that are thought to represent fairly well the total compo- 

 sition of the Cuyamaca Basic Intrusive mass. Dike rocks are excluded. 



PYRRHOTITE 



LABRADORI TE 



LABRADORITE -BYTOWNITE 



PYRRHOTITE 



PYRRHOTITE 



6- 



MAGNETI TE 



ANDESINE, OLIGOCLASE 



'O 20 SO 40 SO 60 70 SO $0 



SUM OF PERCENTS OP PERROMAGNESIAN MINERALS 



Fig. 4. Graphs showing the relation existing between the amount of ore 

 minerals and the amount of ferro-magnesian minerals, in rocks of the Cuyamaca 

 Basic Intrusion characterized by four different plagioelase feldspars. 



Within each set of rocks characterized by a single type of plagio- 

 clase there is a rough direct relationship between the amount of ore 

 minerals and the sum of the amounts of the ferromagnesian silicates. 

 Figure 4 shows this relationship. The sum of the ferromagnesian 

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



