THE DEEEFIELD SHEET. 433 



A fibrous devitrification sometimes affects all the fragments of a slide, 

 each one being now a pale-yellow devitrified glass of a finely tufted or 

 fibrous structure radiating from many centers. The fibers have the same 

 optical properties as do the spherulites. The inclosing glass is more granu- 

 larly devitrified, polai'izing in dots. 



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; its angular boundaries will be sharply defined and the phenocrysts 

 equably disseminated tlu-ough 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 calcite. Acid removes it readily and leaves only a 

 powdery renmant. The outer colorless part is generally de^^trified 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 colorless 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 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-BRECCIA. 



Under the microscope a fragment of the greenish tuff'-like mass, taken 

 20 feet from the base of the bed, was composed as follows: 



The first thing that atti'acted attention was the fine red sand, each 

 grain being covered with iron rust. Where this was in thick masses it was 

 still red in the interior, but on the exterior was black from the recrystalliza- 

 tion of the iron rust by the caustic efi'ect of the melted lava, in which it had 

 been disseminated in threads and sheets. In the interstices between these 

 dark sand portions many minute angular grains of diabase, like that found 

 in the basal bed, were scattered. These had been broken up by an early 

 explosion and earned up from the base with the sand. The whole had been 



MON XXIX 28 



