Minerals and How They Occur; A Book for Secondary Schools and Prospectors

Minerals and How They Occur; A Book for Secondary Schools and Prospectors

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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. 1906 edition. Excerpt: ...layer-like structure also, but the layers are not so regular as those in the limestone, and are more or less finely bent and crumpled. This structure in gneiss and other crystalline rocks is spoken of as "schistosity "--the rocks are said to have a schistose structure. The granite is not arranged in layers, and is called a "massive" rock. Several of these terms are explained more fully on pages 141 to 150. Names Of Igneous Rocks. The igneous rocks may be divided, as we have seen, into two groups--plutonic and volcanic. The members of each group are again subdivided, according to the percentageof silica thej1 contain, into acid, intermediate and basic rocks. The divisions are made approximately as follows: --If a rock contains over 65 per cent. of silica, it is spoken of as an acid rock; if its percentage of silica is between 50 and 65, it is said to be intermediate in composition. Rocks containing less than 50 per cent. of silica are said to be basic. The names given to Igneous rocks, unless they are glasses, depend on the minerals they contain--hence also on their chemical composition--and on their texture, that is on the form and arrangement of their constituent minerals. The following tabular arrangement of the Igneous rocks shows the way in which they are classified according to the description given above. Only the more commonly occurring rocks are shown in the table. The minerals named are those which are found in the more characteristic varieties of each rock. Hornblende, for example, may be replaced in whole or in part by biotite or by pyroxene, and vice versa. The mineralogical composition of the plutonic rocks can be made out by a glance at the table; for example, it is seen that we may have a syenite which is...show more

Product details

  • Paperback | 66 pages
  • 189 x 246 x 4mm | 136g
  • Rarebooksclub.com
  • Miami Fl, United States
  • English
  • Illustrations, black and white
  • 1236607090
  • 9781236607096