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 immedi- 

 ately 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 sec- 

 tion 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 accen- 

 tuates 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 nature of 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 holocrystal- 

 line 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 



