abstracts: geology 439 



ranging in age from Pleistocene to Recent. The igneous rocks are 

 greenstones, intruding the metamorphic schists; gabbros, in stocks and 

 dikes cutting the Mississippian sandstones; and granites, cutting the 

 limestones of undetermined age in the upper part of the Alatna basin. 



Many of the more striking present-day topographic features were 

 produced by the agency of valley glaciers in the recent past. 



Mineralization is confined mainly to the schists and pre-Mississippian 

 rocks and is apparently not related to the igneous intrusives. There 

 is no mining in the region, tho gold has been found in the gravels in 

 the central part of the Alatna valley and in the headward portion of 

 the Noatak, and veins of copper and gold have been reported in the 

 same general region. P. S. S. 



GEOLOGY. — Reconnaissance of the Jarbidge, Contact and Elk Moun- 

 tain districts, Nevada. F. C. Schrader. Bulletin U. S. Geological 

 Survey No. 497. Pp. 162, with maps, sections, and illustrations. 

 1912. 



The districts described are in Elko County in northeastern Nevada, 

 in a region occupied chiefly by Paleozoic sedimentary rocks, probably 

 Carboniferous. These rocks, comprising quartzite, limestone, shale 

 and slate, are folded and faulted and are intruded by Cretaceous (?) 

 granodiorite. This series is capped by Tertiary eruptives, principally 

 rhyolite, overlain by Tertiary lake beds and Quaternary deposits. 



In the Jarbidge district the rocks are principally rhyolite flows with 

 some Paleozoic sediments. The flows are separable into an older or 

 Miocene (?) group and a younger or Pliocene group. 



The principal rocks of the Contact district are granodiorite and the 

 Paleozoic sediments which it intrudes. The granodiorite is surrounded 

 by overlying, outward-dipping Paleozoic sediments with a known thick- 

 ness of about 1600 feet. They are metamorphosed by the granodiorite 

 and both they and the granodiorite'are cut by dikes of various kinds. 



The Tertiary lake beds occur chiefly in depressions in the Contact 

 district where they have a known thickness of 400 feet and are mainly 

 composed of volcanic tuff. In places they are tilted, flexed and gently 

 folded. They are Pliocene and belong to the Humboldt formation. 

 In the Jarbidge Mountains are some Pleistocene glacial accumulations. 



The ores were deposited in at least two periods, one probably Creta- 

 ceous and the other post-Miocene. 



The Cretaceous deposits are chiefly auriferous and argentiferous copper 

 ores and occur mainly in the Contact district in the contact zone around 



