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ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN.
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longulites and trichites of iron oxide. Zirkel figures and describes a similar appearance in the rhyolites of the 40th parallel.[1] He speaks of faint granular lines "which by their fluidal running form a net with a multitude of meshes of oval shape." The meshes are filled by one of two types of crystallization, the micro-felsitic or the spherulitic. The lines suggested to Zirkel perlitic parting. In the ancient lavas of South Mountain the meshes are filled by the micropoikilitic areas or by spherulitic crystallization or by intermediate stages of alteration, that is, spherulites more or less broken up into micropoikilitic areas. In the trichitic spherulites of the modern rhyolites[2] there is an appearance similar to the micropoikilitic mottling, caused by the breaking up of the radiating spherulitic fibers into irregular areas which extinguish differently; just such an intermediate stage between the spherulitic and a completely micropoikilitic crystallization as has been noted in the ancient volcanics. These observations suggest that the micropoikilitic structure represents recrystallized spherulitic growths when it is not the direct results of infiltration and devitirification. In many cases, the crystallization has undoubtedly never been spherulitic, if however, the micropoikilitic structure has been shown to be subsequent to spherulitic crystallization, that is, to the consolidation of the rock in numerous instances in the acid volcanics, selected from widely separated localities in the South Mountain, the presumption favors the secondary origin of the micropoikilitic structure wherever present in these rocks.

Spherulitic structure.—Two sorts of spherulitic crystallization are present in these rocks. They differ in no essential respect but are unlike in appearance. The most numerous spherulites are also the simplest and smallest. They are colorless microscopic spheres, scarcely or not at all perceptible in ordinary light but showing the usual distinct dark cross between nicols. Spheru-

  1. ↑ Vol. VI., Geo. Exp. of the 40th parallel, Fig. 1, Pl. VI., Fig. 1, Pl. VIII.
  2. ↑ Sections of material from the Rosita Hills, Colorado, and of the Obsidian Cliff, Y.N.P., were kindly loaned the writer for comparative study by Dr. Cross and Professor Iddings.
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