GRANITE-PORPHYRY. 221 



although separated by limestone, it seems probable that below the surface 

 the two bodies have a common origin, inasmuch as on top they present 

 nearly identical features. Neither body rises above the general level of the 

 country, erosion wearing away equally both limestone and dike. 



The larger outburst is an irregular body with an east and west trend; 

 the other, a few hundred yards northward, and a short distance beyond 

 the Bullwhacker mine, occurs as a dike from 300 to 400 feet long, with a 

 direction N. 20 W. Both quartz-porphyry exposures are much decom- 

 posed and discolored by oxide of iron. Under the microscope they show, 

 however, orthoclase, quartz and mica, with characteristic isotropic glass. 

 The feldspars are altered into kaolin, with a secondary formation of potash 

 mica; the quartz grains carry both liquid and glass inclusions. In the 

 Bullwhacker mine the porphyry is seen as a dike, striking north 18 W., 

 with a dip 35 to the east. In one place, at least, on the main level it 

 occurs as a white rock, differing from the surface exposure not only in 

 color, but by a relatively large amount of quartz grains and by a develop- 

 ment of numerous modified cubes of pyrites. The pyrites in the dike, in 

 distinction from that carried by the surface body, is perfectly fresh and 

 may account to some extent for difference in color. It is probable that the 

 pyrites is a secondary product, formed after the cooling of the original 

 magma. Careful assays show that the pyrites carries both gold and silver. 

 The quartz-porphyry differs structurally from all other igneous rocks in the 

 region, being distinct from granite on the one hand and from rhyolite on 

 the other. None of the rhyolites at all resemble it. It is possible that it 

 occurs as an offshoot from a large body of granite, the structural features 

 being due to conditions of cooling. No direct evidence of the age of the 

 quartz-porphyry is afforded other than that it penetrates the Pogonip lime- 

 stone of the Silurian. 



Granite-porphyry. The important granite-porphyry bodies are confined to 

 the country lying between the Fish Creek Mountains and the Mahogany 

 Hills. They occur in two large masses separated by a narrow belt of lime- 

 stone with a few smaller outlying exposures, probably offshoots from the 

 parent mass. The principal exposure of this porphyry lies immediately to 

 the west of Wood Cone along the southern base of the Mahogany Hills, 



