Page:The Journal of geology (IA journalofgeology11893univers).pdf/405

This page has been proofread, but needs to be validated.
ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN.
821

scale by Irving,[1] is essentially nothing else than a special phase of the fluidal structure, a phase peculiar to flowage in lava consolidating with extreme rapidity, that is, in an acid glass. The granular crystallization has developed with entire disregard to these curved patches, shreds and stringers.

Lithophysal structure.—Often the macroscopic features of the South Mountain acid volcanics disclose their original character more convincingly than does the microscope. Lithophysæ are one of the structures which are best revealed in the hand-specimen, where they are brought out in delicate relief by weathering. The rose-pink petals of the lithophysæ in a paler pink base produce quite as beautiful specimens of this glassy structure as any rhyolite shows. The micro-pegmatitic structure shows itself in microscopic pegmatoid groups of phenocrysts such as are found in the Yellowstone rhyolites.[2]

Perlitic parting.—That this structure is occasionally present in the South Mountain rocks in great perfection has already been noted. While its presence is a most reliable test of the former character of the rock, its absence furnishes no evidence against the previous glassy condition of the rock, both because many recent rhyolites show no trace of that structure and because it is most readily effaced by devitrification.

Amygdaloidal structure.—In some localities the acid volcanics are conspicuously amygdaloidal. The bright green amygdules of epidote in a pale pink matrix render this rock strikingly handsome. In a few instances[3] the vesicles, whch, as seen under the microscope, are bordered by a broad rim, like the ground-mass in crystallization, but are separated from it by a clear zone of silica and are darkened by an abundance of black iron oxide, bear on the inner edge of this border spherulitic growths. These are surrounded by a clear zone of silica while the center of the vesicle is filled either with an opaque black

  1. ↑ Irving: opus cit., pp. 312-313, Fig. 22.
  2. ↑ Iddings: opus cit., p. 275, Pl. XV., Fig. 5.
  3. ↑ In specimens from Racoon Creek at the east base of Piney Mountain, south of Caledonia Furnace.
This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.