Bulletin - United States Geological Survey Volume 491
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 the residual quartz. The computed mineral composition of a rock is incorrect by multiples of the errors existing in the analysis, and these may be, in fact are, sometimes large. The normative or standard minerals, then, so far as the make-up of a rock is concerned, are partly real and partly conventional. They are, however, quantitative in application and give uniformity to the discussion of rocks. Upon them the quantitative classification is founded. First, all igneous rocks are divided into five classes, which are fixed within certain arbitrary limits by the ratios between the salic and the femic minerals. These classes are as follows: pal 7 I. Persalane: Extremely salic fem 'T pal 7 5 II. Dosalane: Dominantly salic fern l-'_3 sal 5 3 III. Salfemane: Equally salic and femic "lem-1F-5 sal 3 1 IV. Dofemane: Dominantly femic ierab'T sal 1 V. Perfemane: Extremely femic ienT7 That is, the field between an entirely salic rock and one entirely femic is divided into five parts, each representing a definite range of variation. A rock containing more than seven-eighths of salic minerals to one-eighth femic is in the class persalane; one with less than seven-eighths salic to more than three-eighths femic falls under dosalane, and so on. A granite, for example, containing over 87.5 per cent of quartz and feldspar is placed in Class I; a peridotite with over 87.5 per cent of femic minerals belongs in Class V. Many basalts, gabbros, diorites, etc., contain salic and femic compounds in nearly equal proportions, and are therefore in Class III. From the norm of a rock its class can be determined at once, and in many cases a mere inspection of the analysis is sufficient. The two extreme classes occupy each one-eighth of the field; the other...
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