OTHER PORPHYRIES. 83 



ical replacer of the Gray Porphyry on the north and of the Sacramento 

 Porphyry on the east, occurring mainly above the Blue Limestone, but with 

 either White Porphyry or Weber Shales interposed between it and that hori- 

 zon. In California gulch it is also found at lower horizons, but apparently 

 cutting across them upwards. 



MOSQUITO POKPHYRY. 



This porphyry, a light-gray, fine-grained rock occurring exclusively 

 in the form of dikes, is formed of quartz, two feldspars, and biotite. The 

 quartz is very prominent, in clear, irregular grains; orthoclase feldspar 

 is predominant over plagioclase; biotite occurs in small leaves and is not 

 abundant. The occurrence of macroscopical apatite in glistening hexag- 

 onal prisms is a noticeable feature of the rock. The microscope discloses 

 a remarkable association of small ore grains (ilmenite, pyrite, specular hem- 

 atite, and magnetite), together with zircon. 



Occurrence. The type rock was only observed in dikes in the Archean, 

 viz, in the North Mosquito Amphitheater, on the north face of Mount Lin- 

 coln, and in Cameron Amphitheater where it extends from the Archean up 

 into the Paleozoic. 



GREEN PORPHYRY. 



This is a fine-grained, almost compact rock, of light-green color, result- 

 ing from the chloritic decomposition of its original constituents, which renders 

 their identification difficult. Quartz, two feldspars, biotite, and hornblende 

 have been identified; but the relative proportions of orthoclase and plagio- 

 clase are not readily apparent. Muscovite and calcite are decomposition 

 products of the feldspars. The groundmass is often so subordinate that 

 the rock seems macrocrystalline. 



occurrence. It is found as interstratified sheets on lower Loveland hill 

 near the Fanny Barrett claim and in Cambrian quartzite on the north side 

 of Mosquito gulch; also, as a dike running north across the Paleozoic beds 

 from the lower edge of Bross Amphitheater. 



SILVERHEELS PORPHYRY. 



This rock forms important intrusive sheets on the mountain mass of 

 Silverheels outside of the limits of the Mosquito map ; it has not been so 



