220 University of California Publications in Geology [You 13 



nickel or copper. The chalcopyrite and pentlandite generally occur 

 in one of three ways: (1) as grains, whose contacts against the pyr- 

 rhotite are irregular, sometimes serrated, curves; (2) as regular forms 

 which show on the polished surfaces as lath sections with blunt ends ; 

 or (3) as forms which show as narrow wedges on the polished surfaces. 

 The last two may be expressions of essentially the same form. 



There are no veins of pentlandite or chalcopyrite in the pyrrho- 

 tite and there is no relation whatever of the nickel and copper minerals 

 to the parting planes of the pyrrhotite. The pentlandite and chal- 

 copyrite often occur together and then may form a compound grain 

 with a common exterior boundary against the pyrrhotite, similar to 

 the boundary of a compound grain of the three sulphides against the 

 silicates. Almost invariably the nickel and copper minerals are found 

 at the edge of pyrrhotite grains. An exception to the rule are minute 

 tufts of pentlandite which were found in one specimen along a vein- 

 let of calcite within the pyrrhotite. As the calcite is known to be 

 secondary, these tufts are thought to be secondary pentlandite. 



As a further proof that the nickel mineral occurs only in pyrrhotite 

 the results of a series of qualitative tests for nickel with dimethyl- 

 glyoxime may be cited. Three basic rocks that were known to contain 

 no pyrrhotite did not carry enough nickel to give the faintest reaction 

 with this delicate test. On the other hand, some of the pyrrhotite- 

 bearing rocks, including three from the immediate vicinity of the 

 ore body, do not carry nickel, showing that some of the pyrrhotite is 

 not nickeliferous. 



Relation- of sulphides to rock alteration. — Chlorite is of rare occur- 

 rence, but in those few specimens in which it was identified it is clearly 

 of later age than the ore minerals. Secondary green hornblende is 

 much more common than chlorite. The textural relations show that 

 to a large extent at least it also is later than the sulphides. As further 

 evidence of this the following statistics may be cited : 



Of 9 gabbros with green hornblende, 6 carry pyrrhotite, 3 have 

 no pyrrhotite. Of 16 gabbros, both with and without green horn- 

 blende, including the 9 rocks above, 11 carry pyrrhotite, 5 have no 

 pyrrhotite. The ratio of gabbros carrying sulphides to those without 

 sulphides is the same for all gabbros as for those carrying green horn- 

 blende. Furthermore, none of the gabbros and norites with over 1 

 per cent sulphide was found to contain any green hornblende. 



Relation of sulphides to total composition of rocks. — All of the 

 rocks of the Cuyamaca Basic Intrusion carry either magnetite or 

 pyrrhotite. Some of the rocks carry both ore minerals ; but in general 



