418 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



was uniformly toward a center, the surrounding magma closing in about the 

 growing crystals, which must always happen in the case of phenocrysts, 

 though generally by inappreciable stages, owing to the comparative slowness 

 of their growth. 



The origin of lithophysa 3 must be due to the more abundant presence 

 of water vapor in spots in the magma, the greater viscosity of the sur- 

 rounding magma and its generally viscous condition, the Aery rapid 

 crystallization of jointed rods of feldspar and attendant condensation, fol- 

 lowed by the further condensation of the remainder of the mass upon the 

 crystallization of the silica, which must have taken place in the presence 

 of highly heated water vapor. The minerals produced are like those 

 crystallized artificially in closed tubes in the presence of highly heated 

 water vapor. 1 



In certain cases the spherulitic growths appear to be little more 

 than incipient crystallizations, although the spherulites may be megascopic. 

 They are only faintly doubly refracting, and probably consist of extremely 

 minute fibers. Where such spherulites occur in colored banded glass the 

 irregularly twisted bands pass through the brownish- gray spherulites with- 

 out interruption, but their bright colors are changed to brown, with the 

 formation of opaque grains (PI. L, fig. 4). Within such spherulites in 

 some cases there are delicate branching. trichitic needles, radiating from the 

 center, which are probably augitic, besides other delicate curved needles 

 with high index of refraction and strong double refraction, whose character 

 was not made out. 



In some forms of rhyolite the appearance of welded glass fragments 

 or veil structure is retained, although the mass is faintly doubly refracting 

 and may be spherulitic in part, incipient spherulitic needles traversing the 

 rock in various directions without regard to the former lines of flow, which 

 are marked by opaque dustlike particles. In these cases it is evident that 

 the spherulitic crystallization took place after the molten mass had come 

 to rest. 



>Friedel and Sarasin, Bull. Soc. ruineralogie, 1879, vol. 2, p. 158; ibid., 1880, vol. 3, p. 171; 

 Comptes rendus Acad, soi., Paris, 1881, vol. 92, p. 1374; ibid., 1883, vol. 97, pp. 290-294. K. von 

 Chrustschoff, Am. Chemist, 1883; Tschermaks mineral. Mittheil., vol. 4, p. 536. Stanislas Meunier, 

 Comptes rendus Aead. soi., Paris, vol. 93, 1881, p. 737. De Haldat, Aunales chiruie, vol. 46, p. 70; 

 Neues .Takrbuch fur Mineral., 1833, p. 680. A. Daubree, Etudes synthetiques, etc., Paris, 1879, pp. 

 154-179. For a historical review of the theories regarding the formation of lithophysa?, see page 287 

 of the author's paper on Obsidian Cliff, in the Seventh Annual Report of the U. S. Geological Survey. 



