Bulletin - United States Geological Survey Volume 310; No. 338 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 Excerpt: ...be taken as such. The oldest formation forms the border of the lava series and inside of this successively younger flows outcrop, the latest in the center. Local faulting and sheets of outwash deposits have somewhat obscured these relations in places. The Antelope Range lavas occupy a broad syncline pitching to the northeast, while those of the Swett Hills and Harmony Mountains have a general eastward dip and are not folded. PETROGRAPHY. Early trachyte.--The rocks of the early trachyte bed differ considerably in texture in different parts of the district as well as in different parts of the series. The color and mineralogical character, however, remain the same throughout the district. In general the rocks are dark and dense with few phenocrysts, principally feldspar, subordinate biotite. In the Antelope Range, where it is typically exposed, the main part of the formation is a dense dark-red or purple porphyritic trachyte with very few phenocrysts. The phenocrysts are mainly orthoclase, and to a less extent plagioclase, of the variety andesine-labradorite. Both feldspars show Carlsbad twinning. They are altered along cracks and on the borders to quartz and calcite. The groundmass is dense, and in some layers contains amygdules, which are partly or GRAPHIC COMPARISON OF COMPOSITION OF ALTERED ANDESITE, ANALYSIS "e," WITH FRESH ANDESITE GRAPHIC COMPARISON OF COMPOSITION OF ALTERED ANDESITE, ANALYSIS "d," WITH ALTERED ANDESITE, ANALYSIS "c" entirely filled with quartz and chalcedony. Near its base the trachyte in places becomes ferruginous, yielding from 6 to 7 per cent of iron. The upper 30 feet of the bed has a somewhat varying character in the Antelope Range area, being composed of a main layer of lavendergray trachyte, with th...