The Hollow Spherulites of the Yellowstone and Great Britain

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Abstract
I. Introduction In a paper which I had the honour of presenting to the Geological Society some time ago on the Pyromerides of Boulay Bay, I explained the formation of these nodules by supposing the extrusive magma in which they were produced to be imperfectly mixed and in a somewhat viscous state, a conclusion which facts in the field appear amply to justify. These, and other nodules which I studied in North Wales and near Wrockwardine, led me to suppose that the dominant characteristic of these peculiar structures was not that of spherulitic growth, which rather was secondary, and that the principal cause operating in their formation, as just remarked, was a clotting in the magma by a process of flow-brecciation. At the same time, I did not suggest that this was the sole means by which pyromerides were produced, and moreover I purposely left some of the structures which are found, notably the not infrequent concentric arcs of quartz, for later consideration and discussion. Recently, in crossing the American continent, I made a detour to the Yellowstone Park with the object of studying the obsidian there, and it is the result of this excursion which has led to the putting together of the following notes. Since the paper above referred to was written, I have examined the rocks of Boulay Bay and Wrockwardine for the third and second times respectively, and have studied those of Pontesford Hill. The general features of the National Park of the United States are

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