X.—Diagram of Composition of Igneous Rocks

Extract

Recent studies about the average chemical composition of larger numbers of igneous rocks in the aggregate have shown that figures obtained from any one hundred or more samples are very similar, in fact practically equal. (See A. Harker, “On the Average Composition of British Igneous Rocks”: Geol. Mag., No. V, May, 1899.) This will be found also the case when comparing the following average which I calculated from the analysis of igneous rocks compiled by Roth in his “Petrographie der plutonischen Gesteine.”