1922] Hudson: Geology of the Cuyamaca Region of California 221 



it may be said that a rock with considerable pyrrhotite will have little 

 or no magnetite, and those rocks with considerable magnetite will have 

 little or no pyrrhotite. There thus seems to be some kind of a recip- 

 rocal relationship between the two minerals. The most basic rocks 

 are those in which the disseminated sulphides are most likely to be 

 found. As most of these basic rocks carry either augite or hypers- 

 thene, it might be thought that the presence of these pyroxenes favored 



* b ">° Ab 7* A *so ~Z*„ 



Fig. 3. Graph showing- relationship between (1) the chances of finding pyr- 

 rhotite in the various rocks of the Cuyamaca Basic Intrusion, and (2) the kind 

 of plagioclase characterizing the various rocks. 



the development of the sulphides. This, however, is not the case, as 

 none of the hypersthene diorites or augite diorites carries any pyrrho- 

 tite. The kind of plagioclase is the determining factor for the presence 

 or absence of pyrrhotite. The more basic the plagioclase the more 

 likely is the rock to carry pyrrhotite. As the feldspar generally makes 

 up well over half the rock, it may be said that the presence of pyrrho- 

 tite is conditioned by the total composition of the rock. Figure 3 is 



