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University of California Publications in Geology t VoL - 13 



contacts against the pyrrhotite are smooth, clean-cut curves. It some- 

 times occurs in somewhat elongated masses, which might be interpreted 

 as veins, but the chalcopyrite shows no tendency to follow the parting 

 of the pyrrhotite ; and the highest magnification fails to show com- 

 municating channels between adjacent chalcopyrite masses. 



The polydymite (?) is frequently intimately associated with the 

 chalcopyrite and has much the same textural relations to the pyrrho- 

 tite as has the chalcopyrite. It is found in equant grains with rounded 

 outlines and as irregular masses which may send off several branching 

 apophyses between the adjacent pyrrhotite grains. These apophyses 

 terminate as blunt wedges. They do not in all cases confine them- 

 selves to the space between two pyrrhotite grains, but extend into 

 single individuals of pyrrhotite, in no case, however, showing any 

 tendency to follow the parting planes. In one case the parting planes 

 were seen to curve on approaching one of the apophyses of polydy- 

 mite ( ?). 



The examination of numerous polished surfaces failed to show a 

 single case where either polydymite (?) or chalcopyrite occurred with- 

 out pyrrhotite. Furthermore, masses of the nickel or copper mineral, 

 that might be interpreted as veins cutting the pyrrhotite, stop ab- 

 ruptly on reaching the bounding planes between silicates and sulphides. 

 This relation is shown in plate 11, figure 1. In some instances it 

 appears as though the chalcopyrite had been mobile after the poly- 

 dymite had ceased to form. In other instances the reverse relation 

 is shown. It is probable that the copper and nickel minerals were 

 formed at very nearly the same time. Both chalcopyrite and poly- 

 dymite ( ?) were apparently mobile after the solidification of the 

 pyrrhotite. 



The Disseminated Sulphides op the Cuyamaca Basic Intrusion 



A study of thin sections showed that the pyrrhotite occurs in 

 two ways, (1) as irregular grains occupying the space where several 

 silicate individuals meet, (2) as grains with simple oval or circular 

 section, entirely inclosed in single silicate crystals (pi. 11, fig. 2). The 

 most irregularly bounded grains of pyrrhotite are no more complex in 

 their outline than are many augite and brown hornblende grains. 

 While in some of the rocks examined the pyrrhotite occurs for the most 

 part in association with the ferromagnesian minerals, yet in none of 

 the rocks is it restricted to this position and in many of them .the 

 pyrrhotite shows no preference for any one silicate species. 



