BETWEEN WESTON AND MIKE FAULTS. 235 



Pilot tunnels, the latter of which is directly on the fault line. The workings 

 of the various shafts of the Printer Boy mine follow a crack or fissure in 

 the body of this porphyry and cut an apparently included body of Pyri- 

 tiferous Porphyry. The Gray Eagle tunnel in Eureka gulch is on the 

 western limits of the body, in contact with an underlying mass of Pyritif- 

 erous Porphyry. The Fitz-James (?) shaft (M 54), at the head of Eureka 

 gulch and just west of the Mike fault, after penetrating the Wash, was sunk 

 through a large mass of decomposed porphyry, apparently of two kinds, 

 one supposed to be Pyritiferous Porphyry, and reached the White Porphyry, 

 still below that. This body of Pyritiferous Porphyry is apparently part of 

 the main sheet that covers Breece Hill, and seems to thin out to the south 

 and west. It forms the bed of California gulch from Oro City up to the 

 Pilot fault, while the underlying White Porphyry outcrops below Oro City. 

 The shaft L 44, still on the east side of the Mike fault, is sunk in this same 

 underlying White Porphyry. 



Mike mine. The Mike mine, just east of the head of Nugget gulch, is 

 . also sunk in the White Porphyry, a little west of the line of the fault. The 

 porphyry here shows a very peculiar semi-columnar structure, which is evi- 

 dently due to the pressure and movement caused by the fault. It separates 

 out in long, flattened prisms, and the porphyritic structure of the material, 

 which is reduced practically to a clay, is almost lost. The flat surfaces of the 

 prisms are parallel to the fault plane, and not at right angles to it, as would 

 be the case if it were the columnar structure of a dike. 1 



Breece Hill. The whole surface of Breece Hill north of California gulch 

 and east of the Mike fault shows nothing but Pyritiferous Porphyry. In 

 the weathered 1'ock, as has already been stated, pyrites are not generally 

 found, having been dissolved out by surface waters; but wherever it is ex- 

 posed bj shafts or tunnels it is found to contain, at a distance from the sur- 

 face, a most remarkable quantity of fine crystals, varying from almost mi- 

 croscopic size tc one-eighth of an inch or more in diameter. These are fre- 

 quently concentrated along natural joints in the rock, and in such cases 



'Developments made iu this mine since the completion of field- work have confirmed the asser- 

 tions made in regard to tho structure at this point, and shown on Atlas Sheet XVIII, Section FF. The 

 contact was struck in the shaft at a, depth of 426 feet, and the fault proved by a drift run east. The 

 formation dips 20 to tho southwest., showing that the amount of basiuing-up was under rather than 

 over valued. The ore found is principally snlphurets and eaid to be exceptionally rich. 



