Compte Rendu

Compte Rendu

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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. 1893 edition. Excerpt: ...biotite-granite: These rocks present a marked contrast to the last type in their dark color. They always contain green hornblende, biotite, orthoclase, and plagioclase, sometimes one and sometimes the other in excess. Quartz is also usually present and not infrequently rutile, sphene, and epidote as well. Under the microscope they generally show evidence of profound dynamic action. In all probability they represent ancient eruptive masses which have been subsequently greatly changed and recrystallized by earth movements. They are most extensively developed around Georgetown and near Cabin John. In quarries at the former place clearly defined inclusions of other rocks have been noticed, which substantiate the theory of their eruptive origin. Serpentine and steatite: A few small lenticular areas of serpentine and soapstone occur within the area under consideration. They are usually closely associated with the more basic hornblendic rocks, and are, probably, like these of eruptive origin, although this hypothesis can not as yet be considered as definitely proved. Gabbro: Two small elongated exposures, presumably dikes, of traplike rocks, which the microscope shows to be in all respects identical with the Baltimore hypersthene-gabbro,11 occur near West Falls Church station, and a larger area surrounds Bethesda Park, which grades toward the south into the diorite. Broad Branch schists: On the road leading northward from the Pierce Mill road, along Broad branch, a narrow band of thinly foliated sericitic, chloritic, and siliceous schists is exposed. These rocks differ considerably in character and appearance from those about them, but still they grade imperceptibly into the granite and gneiss which lie both on their eastern and western sides. The belt, ..show more

Product details

  • Paperback | 216 pages
  • 189 x 246 x 12mm | 395g
  • Rarebooksclub.com
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236504941
  • 9781236504944