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shape in the hand specimen and their sharply defined outline in the thin section in ordinary light alone testify to their former presence. With crossed nicols these boundaries become inconspicuous, and the field of the microscope shows only a uniform quartz-feldspar mosaic. The crystallization within the spherulitic boundary is sometimes finer grained than that of the groundmass, or the micropoikilitic structure is present in the former when absent from the latter, otherwise the spherulite is in no way distinguished from the groundmass. In the case of the chain spherulites the alteration is complete and universal. There is, in ordinary light, an impressive similarity with the fresh chain spherulites of the Yellowstone Obsidian. The same irregularly scalloped outline, the same central chain of clear spherules. With crossed nicols the close similarity vanishes, for in the ancient rocks the radial growth has utterly disappeared. The clear spherules are composed of finely granular quartz while the sinuous border is not to be distinguished from the quartz-feldspar groundmass.

Axiolitic structure.—Closely related genetically to the chain spherulites, but unlike them in being linearly radial rather than centrally, is the axiolitic formation.^[1] These have been described in rhyolites and occur somewhat sparingly in their ancient prototypes of the South Mountain.

Rhyolitic structure.—The sections in which the axiolites were observed possess a holocrystalline character, but exhibit in ordinary light flow and vesicular structures, together with stringers and shreds and curved patches of a brownish red color forming what has been called a rhyolitic structure. This latter structure, which has been figured and described by Rutley,^[2] Nordenskjöld,^[3] and Vallée-Poussin,^[4] and on a macroscopic