824 THE JOURNAL OF GEOLOGY. 



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 grada- 

 tion 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 ground- 

 mass was, at the time of consolidation, wholly or partly glassy. 

 The secondary character of some of the holocrystalline ground- 

 mass once conceded, and the indications of an original glassy 

 base recognized, it is easy to suppose that the former was devel- 

 oped 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 crys- 

 tallizing 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 



1 Daubree : Gdologie Experimentale, 1879, p. 158. 



2 Vallee-Poussin : Les Eurites quartzeuses (rhyolites anciennes) de Nivelles et 

 des Environs. Bull. Acad. Roy. Sc. Lett, et des Beaux Artes de Belg. 56 annue, 3d 

 series, Tome 13, No. 5, 1887, pp. 521-522. 



3 Brit. Assoc. Rep., 1840. 



* Trans. Roy. Soc. Edin., Vols. XXXIL, XXXIII. 



