On Primary and Secondary Devitrification in Glassy Igneous Rocks

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Abstract
A few prefatory words are needed in explanation of the form of this paper. The authors have frequently discussed its subject, the elder of them having kept it in view since 1877; while the younger has enjoyed favourable opportunities of studying large spherulites, especially those in the obsidian of the Yellowstone, which the other knows only from hand-specimens. When they had agreed upon a joint paper, each wrote a draft, one of them having undertaken to fuse them together. But he found this impracticable; for, while their conclusions were practically identical, the paths followed were very different, so the papers, after substituting cross-references for some passages common to both, are now presented as separate chapters. Part I.—By John Parkinson, Esq., B.A., F.G.S. The excellent account and figures published in the Memoirs of the United States Geological Survey by Prof. J. P. Iddings render superfluous any general description of the well-known spherulites of Obsidian Cliff in the Yellowstone Park. Nevertheless some mention, however brief, must be made of a few facts, as these are closely connected with the general problem of devitrification. Excluding microliths, the first-formed crystallizations are the ‘granophyre-groups’ of Prof. Iddings. These are intergrowths of felspar and quartz built with extreme delicacy, two or more crystals of felspar entering into the composition and forming rectangular or rudely spherical outlines. The greater the number of felspar-individuals the closer is the approximation obtained to a spherical form. As Prof. Iddings states, it is clear that these microscopic ‘granophyre-groups,’ together with the

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