Manual of Geology; Theoretical and Practical Volume 1

Manual of Geology; Theoretical and Practical Volume 1

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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. 1885 edition. Excerpt: remaining traces of the throats up which the igneous matter was ejected, for these outbursts seem analogous to recent submarine eruptions among the Canaries, which rose from the floor of a moderately deep sea, and in a few years were so completely worn away that no definite indications of their position can now be ascertained. In Cader Idris the Arenig rocks include a jointed porphyry, somewhat hornblendic, and therefore possibly andesitic, which is 1700 feet thick, and extends northward continuously to beyond Aran Mowddwy. Upon these lava streams are 300 feet of blue slates, formed of muds which may have been to a large extent derived from the decomposition of volcanic materials; and higher up are 100 feet of porphyritic felspathic ashes, and finally 500 feet of a greenstone, which is full of air cavities, and therefore was poured out under moderate pressure of water or air. Many intrusive sheets of greenstone also occur. Lower down the mountain, the newer beds also alternate with felspathic ashes and lavas, which are rudely columnar, and are about 1500 feet thick; but Sir Andrew Kamsay is doubtful whether the lavas are really contemporaneous, and out of the 3600 feet of igneous materials interstratified with the slates, believes that only 400 feet of ash-beds can with certainty be regarded as derived from craters which were active during the Arenig period. The Arans, however, have the same general structure as Cader Idris, and there is here less room for doubt as to the contemporaneous character of the interstratified igneous rocks, because the lavas have baked the slates over which they flowed, giving them the texture of porcelain, while the slates which rest upon the lavas are unaltered. The volcanic ashes are frequently vesicular, more

Product details

  • Paperback | 270 pages
  • 189 x 246 x 14mm | 485g
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236525299
  • 9781236525291