EHTOLITE SOUTH OF BRIDGE BAT. 385 



hollow. A pitchy black obsidian with bluish metallic luster is filled with 

 minute pores that give it a rough fracture surface, besides some larger 

 pores which are pumiceous and generally occur about the phenocrysts, some 

 of the larger feldspars appearing to have been fractured and dragged apart. 

 Another form of obsidian is so finely vesicular as to be almost a pum- 

 ice; it is dark gray colored, with grains of black glass scattered through it. 

 Others are lighter gray, and the most pumiceous rock is silvery white and 

 fibrous. In the larger cavities the glass has been drawn out to the finest 

 threads, like spun glass. Among the phenocrysts the few light-green augites 

 are plainly recognizable. There is black glass with bright yellow pumi- 

 ceous spots, and sanidine crystals which have split open down the middle 

 lengthwise, and others irregularly cracked and pulled apart, the cracks not 

 having become filled with the glass, which must have been expanding into 

 pumice at the time. This grades into rock in which the yellow pumice 

 preponderates over the black glass. There is perlite of black, red, and 

 brown glass intimately mingled and banded, with feldspars arranged nearly 

 parallel to the planes of flow; also a light-red dense perlite with small 

 black spots. There is a light-red, lithoidal, fibrous, vesicular variety 

 with irregular patches and remnants of black glass, besides light-gray 

 lithoidal rock, with streaks of dark-gray and black glass and porous and 

 scoriaceous portions. Some of these forms of the rhyolite are masses in 

 the breccia, but the more finely brecciated material presents a still more 

 variegated appearance. The most striking breccia is a light-red, also 

 reddish-brown, finely porous glass filled with lumps of light-gray glass of 

 all sizes which is finely porous and minutely crackled, besides others of 

 dark-gray pumice and 'rounded lumps of highly inflated vesicular black 

 glass. Some of the black glass is compact and occasionally spkerulitic. 

 Another form of breccia consists of fragments of black and red obsidian in 

 a matrix of smaller fragments of the same, most of which are red, appar- 

 ently cemented together by a porcelain-like material, which is pink or 

 white. There is distinct evidence of plasticity and flow in the form and 

 arrangement of the small pieces of glass, and the porcelain-like portion is 

 a crystalline modification of the magma, which can be traced into spheru- 

 litic patches. The reddening of the glass seems to have occurred subse- 

 quent to its breaking up, for the fragments of black obsidian have a red 

 margin or surface of variable thickness. 



MON XXXII. PT II 25 



