Page:The Journal of geology (IA journalofgeology11893univers).pdf/403

This page has been proofread, but needs to be validated.
ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN.
819

parallel bands simulating lines of bedding. Sometimes these bands are 4 m. m. wide, at a nearly uniform distance apart and of an indefinite length. In other cases they are very narrow, dwindling into mere lines and dying out, to be replaced immediately by other lenticular bands. The rock cleaves readily parallel to the planes of these bands, which have become planes of weakness and solution, and the spherulites are entirely replaced by secondary silica. This fact, imparting to the bands an opaque white color, render them the more conspicuous in contrast with the blues or reds of the rock surface.

The spherulites which remain unaltered show in the thin section clear cut, circular, semicircular, and fan-shaped outlines, and are colored purple or red by finely disseminated particles arranged either radially or concentrically in threefold zones. Feldspar phenocrysts often occupy the center of the radial growth. These well preserved spherulites are associated with a groundmass which preserves the characteristics of a glass in great perfection, and which, in ordinary light, could readily be mistaken for a fresh glassy lava. It bears the closest resemblance to the base of some of the Colorado rhyolites. Delicate perlitic parting, which because of its delicacy is usually obliterated, is here preserved in wonderful detail. The presence of innumerable globulites accentuates the perlitic and rhyolitic structures. With crossed nicols the aspect of the groundmass completely alters. All glassy structures disappear, to be replaced by granular quartz and feldspar.

It is impossible by any description to carry the definiteness of conviction as to the original glassy natureof the groundmass which the character of such rock-sections justifies. To one who has studied them in both ordinary and polarized light there can be no question as to the secondary character of the holocrystalline groundmass. One cannot escape the conviction that the rock originally consolidated as a spherulitic perlite, and has become holocrystalline by a process of devitrification.

Associated with a groundmass, whose early glassy condition is not so strongly marked, are the altered spherulites. Their spherical

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.