418 



University of California Publications. 



[Geology 



The large quartz phenocrysts (3mm) visible maeroscopically, 

 frequently show in thin section the smoothly rounded outlines 

 characteristic of porphyrinic quartz modified by magmatic corro- 

 sion. In one particular instance, however, the metasomatic alter- 

 ation of the groundmass has also affected the phenocryst. In 

 this case the quartz, which possesses a very nearly perfect hexa- 

 gonal outline as indicated by a peripheral arrangement of inclu- 

 sions, is surrounded by a narrow zone of secondary quartz in 

 optical continuity with itself, but fading irregularly into the 

 quartz of the groundmass. 



The first-class ore at Campo Seco consists of chalcopyrite 

 running about 8 to 10 per cent. zinc. The ore is often banded 

 with black granular sphalerite. The gold content varies from 

 one to three dollars; the silver values are about the same. In 

 spite of the greater losses in the metallurgical treatment with 

 high zinc content, it is found that the gold and silver values 

 increase with the amount of zinc present. 



The ore bodies are lenticular in shape, and attain a maximum 

 thickness of 30 feet. 



COPPEROPOLIS,* CALAVERAS COUNTY. 



The meta-andesite belt attains a great thickness here, and 

 displays in unusual perfection the evidence of its extrusive 

 origin. Various amygdaloids, coarse breccias, and tuffs graduat- 

 ing into elay slates largely preponderate over, the massive por- 

 phyries. In addition to the angular blocks of andesite, and at 

 times fragmental quartz porphyry, the breccias are often thickly 

 crowded with granitic material — hornblende gneiss, diorites, and 

 hornblendites. The intimate association of the clay slates with 

 the finer pyroclastics suggests a possible submarine origin for 

 the entire series. t Dynamic movements have affected the region, 

 shearing the massive volcanics but slightly, while often imposing 

 a very thorough schistosity upon the finer tuffs, parallel to the 

 bedding. 



The Copperopolis Lode, which has been traced over a length 

 of 10,000 feet, occupies a rather persistent horizon of clay slates, 

 microerystalline chlorite schists, and chlorite schists whose width 



* J. D. Whitney, Geology of California, vol. I., pp. 254. 

 t A. Geikie, Textbook of Geology, vol. I., pp. 339. 



