Bulletin Volume 472-480

Bulletin Volume 472-480

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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1911 edition. Excerpt: ...of Henson Creek and gradually thin out eastward. They have not been found east of Nellie Creek. These andesites occur in flows varying from a few feet to more than 100 feet in thickness. Below the Burns tuff occur nearly identical sheets of rock, which have not been distinguished on the map. In several places it is clear that these lower bodies have been intruded, and they no doubt belong to the same epoch of eruption as the flows above the tuffs. In El Paso Gulch and the ridge east of it the evidence of intrusion is plain and is expressed in part on the geologic map. The pyroxene andesites are dark porphyritic rocks, with numerous crystals of augite and plagioclase visible to the unaided eye. They are massive except in the upper zones of most flows, where a pronounced vesicular texture is commonly developed. Secondary quartz and bluish chalcedony are usually found in these vesicles and, as they readily weather out of the rock in exposed places, small nodules or fragments of them strew the ground in many places near outcrops. The microscope shows that hypersthene was a former constituent of these rocks, its place being now taken by serpentine, chlorite, and other alteration products. Quartz and orthoclase are generally present in small amount. HENSON TUFF. The last member of the Silverton volcanic series is a tuff much like the Burns latite tuff. Its greatest development is in the valley of North Fork of Henson Creek in the Ouray quadrangle. It lies on the pyroxene andesites in the region west of Nellie Creek, but is not found east of that stream. The Henson tuff is made up chiefly of d6bris of pyroxene andesite, but contains particles of other volcanic rocks; locally it contains angular fragments of rhyolite, latite, and andesite. There...show more

Product details

  • Paperback | 398 pages
  • 189 x 246 x 21mm | 708g
  • Rarebooksclub.com
  • United States
  • English
  • black & white illustrations
  • 1236775546
  • 9781236775542