EXPLANATION OF PLATE 10 



Fig. 1. Same field as figure 6, Plate 9, but with crossed Nicols. 



Fig. 2. Thin section of hypersthene diorite. X 25. Crossed Nieols. 



The texture is typical of the rocks of the Cuyamaca intrusive. 



Fig. 3. Photograph of thin section of ore from Friday Mine. X 25. 



Crystal of brown hornblende enclosed in sulphides. The dark substance in 

 the center of the hornblende is chlorite, with which is associated actinolite in 

 white ' ' needles. ' ' A rim of fibrous calcite surrounds the brown hornblende. 



Fig. 4. Photograph of thin section of ore from Friday Mine. X 25. 



Brown hornblende in sulphides. 



Fig. 5. Massive ore. X 12.5. 



Pyrrhotite shows parting cracks. Polydymite shows cubic cleavage. Chalco- 

 pyrite distinguished by high luster and freedom from cleavage. Dark gray patches 

 are gangue minerals. 



Fig. 6. Enlarged view of center of field shown in figure 5. X 25. 



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