1922] Hudson: Geology of the Cuyamaca Region of California 219 



A study of polished surfaces of norites and gabbros has modified 

 only slightly the ideas as to textural relationships gained from the 

 study of thin sections, excepting that it was found that the pyrrho- 

 tite occasionally occurs in veinlike masses, as illustrated in plate 11, 

 figures 3 and 4. As such minute veinlets of sulphide were found at 

 onty two places in the nine polished surfaces studied, this phenomenon 

 must be considered exceptional. The typical textural relations of ores 

 to silicates in the basic rocks are shown by plate 11, figures 5 and 6, 

 and plate 12, figures 1-5. 



Pyrrhotite is readily identified in hand specimens of the rocks by 

 its white to bronzy color, high metallic luster, and conchoidal fracture. 

 In the polished surfaces it appears white to pinkish and the traces of 

 parting planes permit it to be readily distinguished from the other 

 sulphides. 



Chalcopyrite sometimes occurs in large enough masses to be made 

 out by the hand lens, but the greater amount is found in such minute 

 individuals that the examination of polished surfaces with the micro- 

 scope is necessary to differentiate it from the pyrrhotite in which it is 

 invariably imbedded. 



A white mineral, with hardness intermediate between pyrrhotite 

 and chalcopyrite, is found in many of the rocks. It takes a high polish, 

 shows no cleavage or crystal outlines. Michrochemical tests made of 

 this mineral are as follows : 



HN0 3 dilute — rich yellow-brown stain, no effervescence. Rubs off readily to 

 untarnished surface. 



HN"0 3 cone- — similar action, but slight brownish stain persists after consider- 

 able rubbing. 



HC1, both dilute and cone. — no effect. 



Aq Eeg. — no effect. 



Samples of pentlandite from Sudbury gave similar reactions, but 

 differ from the Cuyamaca mineral in having a yellow color in com- 

 parison with pyrrhotite. Enough material for blowpipe analysis or 

 quantitative tests could not be isolated, but the evidence given above 

 seems enough to justify the conclusion that the mineral is either pent- 

 landite or else some very closely allied species. 



Relationships between the sulphide minerals. — The pentlandite and 

 chalcopyrite never occur except as portions of a compound grain with 

 pyrrhotite. No matter what the shape of the pentlandite or chalcopy- 

 rite grain imbedded in the pyrrhotite may be, the exterior boundary 

 of the whole sulphide mass is always made up of smooth curves, differ- 

 ing in no way from the boundaries of pyrrhotite grains that carry no 



