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The geology of a section of the Great ~asin in the vicinity of Pioche, Nev., has been studied by memb~rs of the United States Geological Survey, and a brIef summary of the results, by Westgate and Knopf,! has already been published. The writer had the pleasure of serving in the district with Professor Westgate during two field seasons. The part of the area here described lies in the southern part of the Bristol Range quadrangle and the northern part of the Highland quadrangle. (See fig. 11.) Extending through these quadrangles from north to south is a high range of faulted but gently dipping Cambrian limestone, called in the north the Bristol Range and south of a pass known as Stampede Gap the Highland Range. At the north end of ~he Bristol Range occurs an overthrust block of Devo~an sediments, which is known also in a small range 4 miles to the west, where it overlies Miocene (?) lavas. In the vicinity of Blind Mountain, in the souther~ part of the Bristol Range quadTangle, and on the western slopes of the Bristol Range, a down-faulted s~ction of this Devonian overthrust lllass occurs, there mvolved with the Miocene (?) lavas and intruded by a quartz monzonite mass, which has caused extAnsive contact metamorphism not only of the.overthTust mass .but of the Cambrian rocks adj acent to it. This quartz monzonite is the only large intrusive mass exposed in the Pioche district, and its outc~ops are confined within an area of about 2 square miles, although the desert wash filling the valley west of the Bristol Range probably buries a considerable mass of it. 'l'he exposures are considered to represent an upper cupola of a large batholit~ which underlies. the whole Pioche area and from whIch the ore solutIOns emanated. The large size of the batholith is indicated by widespread marmarization of certain beds of ~he limestone and the wide distribution of the ore depOSIts, of zones of intense contact metanlorphis~n, and of lamprophyre dikes. . The exposed igneous rock has two principal facies, but this study has shown that they are related. Both illustrate the type of differentiation described by Fenner,2 in which the character of the magma during

Petrography of the Pioche district, Lincoln County, Nevada