1922] Hudson: Geology of the Cuyamaca Region of California 225 



also taken as proof that the sulphides existed as drops of liquid 

 during the crystallization of the silicates which now inclose them. 



There is a lack of evidence as to the relative time of formation of 

 the three sulphides. They perhaps solidified simultaneously in their 

 present form, or, what is more likely, by analogy with certain metal- 

 lurgical products, 23 we may conceive of the sulphides solidifying as 

 a homogeneous matte, which later became unstable, due to decrease 

 of temperature, pressure, or both, and broke down to the mixture of 

 three minerals as we now see them. 



Relation of the Massive Ore Body to the Enclosing Rocks 



The average composition of the Cuyamaca Basic Intrusion, leav- 

 ing out of account the rocks of the Friday Mine, is approximately 

 that of a gabbro-norite, whose plagioclase is a labradorite, carrying 

 38 per cent albite molecule. If, further, the diorites are eliminated 

 from the calculation, for the reason that they may represent later 

 intrusions and not differentiates in place, the approximate average 

 composition of the mass is that of a gabbro-norite with a labraclorite- 

 bytownite (29 per cent albite molecule) as its feldspar. The average 

 rock of the Friday Mine is an olivine norite, whose feldspar is by- 

 townite (24 per cent albite molecule). It is evident, then, that the 

 rocks adjacent to the massive ore body are more basic than is the 

 average of the rocks from other portions of the intrusive mass. 



Of eighteen basic rocks collected from the Friday Mine workings, 

 only one, an augite diorite, lacks pyrrhotite. More than one-third of 

 the rocks from other localities, excepting the rocks of the two diorite 

 areas, carry no sulphide. Furthermore, the average sulphide content 

 of the rocks of the Friday Mine, excepting those in immediate prox- 

 imity to the ore body, is 1.1 per cent, while the average for the basic 

 rocks from other localities, leaving out of account those which carry 

 no pyrrhotite whatever, is only 0.7 per cent. It is seen, then, that 

 with the increase in basicity of the igneous rocks in the vicinity of the 

 ore body there is also an increase in sulphide content. 



The relations set forth above would lead one to expect gradational 

 contacts between the massive ore and the norite. As has been noted 

 before, the greater part of this contact is determined by slip planes ; 

 but at one point an unslipped contact is preserved. Here the trans- 

 ition from norite, carrying a few per cent of sulphides, to ore carrying 

 over 50 per cent of sulphides takes place within a distance of less than 

 one centimeter. 



23 See pp. 240-241. 



