316 University of California Publications. [Geolouy 



dinate proportion of ealeite, in which are imbedded irregular 

 grains and idiomorphic crystals of quartz. Associated with the 

 calcite is a biaxial mineral with lamellar structure, a refractive 

 index less than that of quartz, a double refraction about that 

 of quartz or less, a pronounced cleavage and a twin structure, 

 which is thus identified as gypsum. Where the garnets prevail 

 the structure is miarolitic ; where the calcite prevails the garnets 

 occur in nests and stringers through it. 



At another locality on the same contact a yellowish garnet 

 rock occurs with drusy surfaces, in cracks, of small yellow gar- 

 nets and small veins of calcite. The garnet in this case has a 

 zonal structure and is doubly refracting. Nearly colorless epi- 

 dote occurs in aggregates of grains enclosed in the garnet. 



Just to the west of Copper Flat and at Pilot Knob occur 

 considerable bodies of iron ore which seem to have the same rela- 

 tion to the porphyry that the iron ore at the Ruth does, although 

 the porphyry, particularly at Pilot Knob, has been well removed 

 by erosion from the immediate vicinity of the ore. The iron ore 

 at Pilot Knob is a black to metallic gray magnetite with a some- 

 what vitreous luster. It is compact and shows no granular struc- 

 ture. Thin sections are quite opaque except for a few areas and 

 veins translucent to reddish yellow light. This is the largest 

 body of iron ore observed in the district. 



It is noteworthy that all the observed occurrences of this 

 contact garnet rock and iron ore are on the lower side of the 

 porphyry mass. It is also to be remarked that, while these pro- 

 ducts are regarded as the result of contact metamorphism, they 

 can in no sense be ascribed simply to thermal metamorphism in 

 the same way that common limestone is converted into marble, 

 or a clay state to an andalusite hornfels* without change of bulk 

 composition. It is evident that they are the products of a reac- 

 tion between the intrusive porphyry and the adjacent limestone. 

 The lime of the garnets was doubtless derived from the lime- 

 stone, but the carbonic acid was eliminated and the silica must 

 have been derived from the porphyry. The most probable means 

 whereby the reaction was effected was aqueous solutions ema- 

 nating from the porphyry and, this being the case, the process 



* e.g., The contact zone at Barr Andlau described by Kosenbusch. 



