This excellent article is an ingenious approach to a very pertinent problem. However, there are many pitfalls in the computations of “norms” which should be pointed out. In this computation, it is largely a matter of chance that there is any resemblance between the calculated minerals—the “norms”ȁand of the actual minerals—the “mode.” It should be noted that the physical properties of a rock are determined by the mode rather than the norm. In Clark's Data of Geochemistry (Frank Wigglesworth Clark, The Data of Geochemistry, U.S.G.S. Bul. 770 (1924), pp. 432) the mineralogic irregularities introduced by the calculations of norms are illustrated by Table I.

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