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 

 rock 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 a 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 bo stone on cooling if the process were gradual 

 enough. 



Figures 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 microliter, (or 

 microscopic minerals), called trichites (from the Greek thrix, 

 hair), such as are common in all obsidians. Fig. 5 shows 

 the texture of a variety of pearlite, a light gray rock of 



5. 



ii m 





5 



Trichites in ob- 

 sidian. 



i m 



Trichites and Fluidal 

 texture in Pearlite. 



Microlites in a Pitchstone 

 from ^Yeisselberg. 



pearly lustre from the Montezuma Bange in the Xevada 

 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 



