Page:The Journal of geology (IA journalofgeology11893univers).pdf/408

the Obsidian Cliff, were completely vitreous save for spherulitic and lithophysal crystallization. In other localities the lava was lithoidal, and in the central portion of thick flows holocrystalline. In this way three types of acid volcanics would be developed—rhyolites, lithoidal rhyolites, and quartz porphyries. Every gradation between these types would accompany them. Thus, while there are certain areas in the South Mountain, notably the Bigham Copper Mine and Racoon Creek localities, which exhibit typical ancient rhyolites, other regions display genuine quartz-porphyries. While in the latter rocks, which constitute a large part of the acid volcanics, the groundmass may have been, and probably was, originally holocrystalline, as in some modern lavas; in the case of the former rocks, it is supposed that the groundmass was, at the time of consolidation, wholly or partly glassy. The secondary character of some of the holocrystalline groundmass once conceded, and the indications of an original glassy base recognized, it is easy to suppose that the former was developed from the latter by a process of devitrification.

That the process of crystallization does not necessarily cease with the solidification of a rock is well known. That the crystallizing forces are active in a glass as well as in a molten magma has been proven by experiment.^[1] This action is exceedingly sluggish, and requires, unless accelerated by heat and moisture, an immense amount of time. Devitrification has been considered the result only of dynamic action.^[2] While dynamic action undoubtedly accelerates the process of devitrification, if it does not initiate it, devitrification may also take place independently of dynamic action, as was the case in the famous example of the old cathedral window-glass^[3] and the ancient devitrified glass from Nineveh investigated by Sir David Brewster.^[4] The nature