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University of California Publications in Geology [Vol. 13 



various figures representative of the composition of the silicate portions 

 of the rocks. The graphs obtained were extremely irregular and 

 seemed to condemn the theory. Later it was discovered that by plot- 

 ting the difference between the percentages of pyrrhotite and magne- 

 tite against the sum of the ferromagnesian minerals irregular graphs 

 could be obtained, which when "averaged" became smooth curves. 

 These curves (fig. 5) show that the difference between the amount 

 of sulphides and that of magnetite, in rocks characterized by any 

 particular plagioelase, stands in direct ratio with the sum of the 

 per cents of the ferromagnesian minerals. 



The origin of the disseminated sulphides. — From the standpoint 

 of the textural relations of the three sulphides, considered as a unit, 

 toward the silicate minerals which inclose them, it is almost incon- 

 ceivable that the pyrrhotite, with its attendant nickel and copper min- 

 erals, could have been introduced into the rocks after the consolidation 

 of the silicates. The reasons for this conclusion are : ( 1 ) lack of 

 veins; (2) sulphides do not occur along cleavages of silicates; (3) 

 contrast between the simple spherical or ovoid grains inclosed in single 

 silicate crystals and the irregular grains with ramifying apophyses 

 that occur at the meeting place of several silicate crystals; (4) irreg- 

 ular grains are no more complex in outline than many of the brown 

 hornblende and augite individuals; (5) the sulphides do not replace 

 the silicates. This is shown by (a) textural relations, (6) lack of 

 preference for association of sulphides with any one silicate or group 

 of silicates, and (c) the occurrence of sulphides bears no relationship 

 to rock alteration. 



With the exception of the minute tufts of pentlandite which were 

 found along calcite veinlets within the pyrrhotite, the pentlandite and 

 chalcopyrite could not have been introduced from without after the 

 solidification of the pyrrhotite. This is shown by: (1) the absence of 

 veins of pentlandite or chalcopyrite in either silicates or pyrrhotite ; 

 (2) the independence of particles of pentlandite or chalcopyrite of 

 the parting planes of the pyrrhotite; (3) occurrence of nickel or 

 copper mineral only as a part of a compound grain with pyrrhotite. 



Conclusions. — The sulphide minerals are essential constituents of 

 the igneous rocks in which they are found. The lack of crystalline 

 outlines on the part of the pyrrhotite and the ramifying apophyses 

 of the more irregular individuals show that the sulphide was the last 

 constituent of the rock to solidify. The simple, round forms of those 

 pyrrhotite grains that are inclosed in single crystals of silicates are 



