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oxide or with granular quartz. Crossed nicols show that the spherulites are oriented optically with the surrounding silica, and that the preservation of the radiate structure is due to the arrangement of impurities. The appearance of these vesicles is very like those figured by Professor Cole,^[1] who explains their formation by a dual mode of growth—a growth from the groundmass outward converging toward a center, as well as from the center. Whatever may be the facts with reference to the Roche Rosse Obsidians, it is not necessary to call into play an abnormal method of crystallization to explain the phenomena observed in the South Mountain rocks. The spherulites projecting into the vesicles, with their bases sunk into its wall, were recognized by Professor Iddings, who kindly examined the section, as tridymite spherulites, such as form on the walls of vesicular cavities in all modern lavas.

Taxitic structure.—Still another structure which the South Mountain rocks possess in common with rhyolites is what has been called the taxitic. This consists in the intimate mingling of two portions of the magma, which, from some cause (liquation), are slightly differentiated. The iron constituent, which evidently separated out in the original glass, has been still further crowded into bands and curved lines by the secondary crystallization. The result is the production in some cases of an irregular mottling: ataxites; and in other cases of a more or less complex network of interlacing bands following lines of flow: eutaxites. This mottling and banding is rendered the more striking by a marked contrast in color. The body of the rock is light gray or pink, and the lines dark blue, gray or red, according as the iron is more or less oxidized. When the iron constituent is arranged in oval or spherical outlines, denoting the former presence of spherulites, the rock may properly be termed a spherotaxite.^[2]