416 DESCRIPTIONS OF ROCKS. 
tals contain small crystals of mica distributed in one or 
more layers concentric with the sides. 
The degree of coarseness in the texture of a crystalline 
Yock has been determined chiefly by the rate of cooling, 
in connection with the nature of the material. Relatively 
rapid cooling produces a fine texture or grain, and very slow 
cooling @ coarser. — 
A melted rock may cool too rapidly to become stony 
throughout, or to become stone at all; and, in the latter 
case, the material made is glass. Common melted glass 
would be stone on cooling if the process were gradual 
enough. 
Iigures 4 to 6 represent much-magnified views afforded 
by transparent slices from glassy rocks, in three of their 
stages between the pure glassy and the true stony state. In 
4, from obsidian, or volcanic glass, of Greenland, there 
are radiating clusters consisting of hair-like microtites (or 
microscopic mincrals), called trichites (from the Greek thria, 
hair), such as are common in all obsidians. Fig. 5 shows 
the texture of a variety of pearlite, a light gray rock of 


Trichites in ob- Trichites and Fluidal Microlitesina Pitchstone 
sidian. texture in Pearlite. from Weisselberg. 
pearly lustre from the Montezuma Range in the Nevada 
Basin, as figured by Zirkel; in this. trichite clusters. besides 
being very numerous, are arranged in lines or planes, and 
some of the trichites are powdered with pellucid grains, or 
globulites, which are incipient crystals. Zirkel represents 
another kind in which the radiating trichites are each a 
string of globulites. Fig. 6 represents a pitchstone from 
