196 Pirsson — Microscopical Characters of Volcanic Tuffs. 



covers they have been pulled out ; that is, they are the same 

 as e on a larger scale. This is shown in f fig. 1, where the 

 vesicles are elongated. A transverse section of this does not 

 exhibit the elongation, but appears as in d. If the elongation 

 is very pronounced the particle may resemble a bundle of 

 fibers, as in A, fig. 1. A transverse section of this shows the 

 ends of the fibers in a berry-like grouping; g, fig. 1. 



Section of a Vitric Tuff. — A typical example of a vitric 

 tuff from a rhyolitic magma, which exhibits well the various 

 forms of glass particles mentioned above, is shown by the 

 drawing in fig. 2. This tuff is relatively little altered in that 



Fig. 2. 



Fig. 2. Ehyolitic vitric tuff from Checkerboard Creek, Castle Mountain 

 Mining District, Montana, showing typical vitrociastic structure. Actual 

 diameter of field 2 mm. 



the glass shards are unchanged in substance and do not act on 

 polarized light. The interstitial material between them is in 

 some cases more minute fragments of glass, while much of it 

 appears by transmitted light excessively tine granular and 

 largely of a brown color ; the nature of the latter in part is 

 indeterminable and it may consist of altered glass dust, and 

 in large part it is chalcedony, deposited hydrous silica, in 

 numerous areas and patches, as shown by its refractive index, 

 double refraction and fibrous radiating nature ; which causes 

 it to yield between crossed nicols, on revolving the plate, a 

 stationary black cross like that of as pherulite. This chalced- 

 ony has a rich brown color by transmitted light. While the 

 tuff in the hand specimen, like so many others, is colored 



