1922] Hudson: Geology of the Cuyamaca Region of California 215 



found particularly easy entrance along the contacts between the schist 

 and basic intrusive and also along planes of schistosity within the 

 schist. Much of the schist on the lower level is so saturated with 

 pegmatitic material that it may be called injection gneiss. 



Calkins has reported that the massive ore body "is cut by a thin 

 dike of pegmatite containing conspicuous crystals of common black 

 tourmaline." 21 The writer was unable to find the locality at which 

 this phenomenon was observed, and believes that the ore has been 

 stoped out there. However, pegmatite was seen cutting a dike of the 

 fine-grained brown-hornblende norite. As these basic dikes are known 

 to be younger than the ore, the later age of the pegmatite also is estab- 

 lished, and Calkins' observation confirmed indirectly. 



The boundaries of the ore. — "With the exception of one boundary 

 plane exposed on the upper level where the ore body is in contact with 

 schist, the sulphide body is completely inclosed in the rocks of the 

 basic intrusive mass. The shape of the ore body (see fig. 2) indicates 

 that the contact between ore and schist is a fault. This contact is a 

 smooth plane along which a small amount of limonite-stained gouge 

 has been developed. Most of the contacts between ore and gabbroic 

 rock are sheared, but a portion of the southern boundary plane, ex- 

 posed in the upper level of the mine, is not affected by shearing and 

 shows a gradation between coarsely crystalline pyrrhotite, carrying 

 relatively small amounts of silicate minerals, and norite, carrying dis- 

 seminated particles of sulphides which differ in no way from the 

 sulphides found in the basic rocks in other parts of the Cuyamaca 

 region. 



MINEEALS OF THE MASSIVE OEE 



Pyrrhotite. — This is the predominant mineral of the ore body. It occurs in 

 individuals without crystal outline, but with well developed parting planes. 

 The extent of these planes shows that the pyrrhotite is coarsely crystalline, 

 some individuals being over one and a half inches in diameter; but the average 

 is less than one-half inch. The parting planes show a dull gray color, but fresh 

 surfaces obtained by breaking the mineral across the parting have a conchoidal 

 fracture and a metallic white to bronze color. In polished surfaces of the ore the 

 pyrrhotite is readily determined from its prominent basal parting, as shown in 

 plate 10, figures 5 and 6, and plate 11b, figure 1, and its pinkish color. 



Magnetite. — Euhedral octahedrons of magnetite are found imbedded in the 

 pyrrhotite. 



Chalcopyrite. — Grains of chalcopyrite up to one-eighth inch in diameter may 

 be seen. It also occurs in smaller grains and minute veinlike masses. On pol- 

 ished surfaces chalcopyrite is identified by its brilliant luster, freedom from 

 cracks and cleavages, and its characteristic yellow color. 



si Calkins, F. O, U. S. Geol. Surv., Bull. 640, 1916, p. 81. 



