SURFACE GEOLOGY. 9 



surface. What its underground character may be is not known, as there 

 have been no explorations made in it. 



Quartz-porphyry. — Quartz-porphy ry appears in two places north of Adams 

 Hill. Mr. Hague assigns no definite age to this rock, but states that it is 

 post-Cambrian. From the manner of its occurrence in the Bullwhacker 

 mine it would appear to be of earlier origin than the ore. This rock has a 

 reddish color on the surface and a granular texture. Where exposed under- 

 ground it is white, shows considerable quartz, and contains cubes of pyrite. 

 Neither variety is hard. 



Rhyoiite. — Rhyolite isabundant in the neighborhood of the mines as well as 

 in immediate proximity to the ore. In some portions of the district it covers 

 large areas, but in the mines it is only found in the form of dikes, which, so 

 far as is known, have never exceeded 20 feet in width. There are particu- 

 larly large outbursts of this rock at Purple Mountain near Ruby Hill and 

 at Pinto Peak. It is of a nearly white color, sometimes with a pinkish 

 tinge, and of various degrees of hardness. 



Hombiende-andesite. — Homblende-andesite occurs near Hoosac Mountain, 

 where it covers a considerable territoiy. It is of a crystalline texture, dark 

 color, and is considerably weathered. The last two rocks are assigned by 

 Mr. Hague to the Tertiary age. 



Peculiar formation in the Phcenix mine. — In the Phoenix mine there is an occurrence 

 of a peculiar rock, the exact nature of which has not been determined. 

 The position which it occupies can be seen on referring to vertical section 

 No. 3, Plate V. As far as known it lies wholly in the quartzite. It is 

 usually of a black color and contains large quantities of magnetite and 

 pyrite. It is everywhere penetrated by small seams of calcite, and some 

 specimens are composed almost entirely of that mineral and claj-, which 

 latter substance often fills cracks and fissures in the mass of the rock. 

 Some pieces showing the least decomposition when treated with boiling 

 chlor-hydric acid give off a great deal of carbonic acid, and the iron and other 

 soluble substances are completely dissolved, while a white skeleton of some 

 siliceous material is left which exhibits a cellular structure. The mass 

 shows no signs of any stratification, and everywhere exhibits evidence of 

 the extended metamorphism to which it has been subjected. The form in 



