ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN. 823 



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 adequately 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 char- 

 acter 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 crys- 

 tallization 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 com- 

 parable to similar flows as they now appear in the Yellowstone 

 National Park. Certain portions of the flow, as in the case of 



1 S. Allport : On certain ancient divitrified Pitchstones and Perlites from the 

 lower Silurian District of Shropshire. Q. J. G. S., Vol. XXXIII., p. 449. 



O. NORDENSKJOLD : Opus cit. 



R. D. Irving : opus cit. p. 312. 



