Page:The Journal of geology (IA journalofgeology11893univers).pdf/407

Trichitic structure.—The universal presence of globulites, trichites and microlites of black and red iron oxide, in flow bands, or indifferently distributed, or in concentric zones around spherulites and vesicles is worthy of mention as a further point of resemblance to the modern rhyolite. Such trichites in similar rocks have been described by various petrographers.^[1] Such, in brief, is the character of the evidence for the secondary nature of some of the holocrystalline groundmass of the acid volcanics of the South Mountain. It is not easy to present the proof so that it shall carry the weight which justly belongs to it. Very much depends upon effects which it is impossible to reproduce by description, but which carry conviction to the student of these rocks. The contrasting appearance of the sections in ordinary and polarized light cannot be adequatly reproduced. The disappearance under crossed nicols of rhyolitic, perlitic, spherulitic, and fluxion structures, so clearly indicated in ordinary light, and their replacement by a homogeneous holocrystalline mosaic is one of the strongest evidences of the secondary character of the crystallization. Nor are there lacking instances where the subsequent nature of the crystallization is in other ways distinctly proven, as in the replacement of radial crystallization by the granular aggregate of quartz and feldspar, which is homogeneous with a granular groundmass, as well as in the character of the micropoikilitic structure. One or more of the structures which have been described are invariably present in the acid volcanics of certain localities. The occurrences, where their structures are absent, show a genetic relationship in the field to typical representatives of the modern rhyolite.

The writer considers that the acid lava flows in South Mountain were, at the time of their consolidation, quite comparable to similar flows as they now appear in the Yellowstone National Park. Certain portions of the flow, as in the case of