SPHERULITES, GLASS, AND SAND. 75 



The glass sometimes undergoes a peculiar calcification, which seems to 

 me rather a metamorphic change produced by the heated waters than a 

 later decomposition by cold atmospheric waters. A fragment of glass 

 will be red brown at the center, pale brown farther out, and perhaps 

 colorless at its border (see plate 9, figure 2). Its angular boundaries will 

 be sharply defined and the phenocrysts equably disseminated through 

 the whole, and with common light the whole seems unchanged glass. 



It will, however, polarize in whole or part in broad patches of bright 

 and softly blended colors and show everywhere the uniaxial figure of cal- 

 cite. Acid removes it readily and leaves only a powdery remnant. The 

 outer colorless part is generally devitrified in plumose patches or in series 

 of minute fibrous globes in the greenish fibrous devitrified glass. The 

 calcite disappears rapidly with acid, leaving an opaque white granular 

 residue, while the white glass becomes opaque white in lines and streaks 

 showing a concealed fluidal structure. 



It is noteworthy that among all the reactions carried out here so little 

 quartz is set free. Under the influence of the heated and carbonated 

 water the glass, rich in calcium and other alkalies and poor in silica, tends 

 to split into calcite and acid feldspars. This explains the formation of 

 spherulites and the fibrous devitrification of the glass, with the abundant 

 development of calcite. 



Glass and sand. — The above glasses came from the top of the breccia 

 bed, where the water or water vapor penetrated without carrying any of 

 the impurities derived from the sandstone. Below this, in specimens 

 taken from the lower portion of the bed, the glass is mixed in every con- 

 ceivable way with the sand. Large patches of the glass are exactly like 

 the deep red brown glass mentioned above, but when examined with 

 polarized light are seen to be made up of broad patches of calcite. 



These glasses, where they come in contact with sand (plate 9, figure 2), 

 show often thin bands of alteration of the central red glass, first, outside 

 the red glass itself, a pale yellow non-polarizing layer, then a pale yellow 

 or colorless layer, polarizing in white lines and dots, and, lastly, a color- 

 less, narrow, highly refracting band, which stands out like a cord between 

 the glass and the sand. At times this band is broken and the parts 

 moved, and the white or pale yellow glass is continuous outward as a 

 cement uniting the sand grains together. This narrow cord-like band 

 then often encircles each of the grains, leaving apparently empty inter- 

 spaces, as one sometimes sees the pebbles of a conglomerate cemented 

 by calcite or silica (see plate 9, figure 1). Treated with boiling HC1 and 

 concentrated boiling HKO, the green central part of the glass where it 

 was thickest between the grains seemed to be in part removed, and 



XI— Bull. Geol. Soc. Am., Vol. 8, 1896 



