THE ORIGIN AND DESCENT OF ROCKS. 



407 



The first stages of crystallization. — The microscopic study of the 

 volcanic glasses reveals great numbers of minute forms known as crystal- 

 lites, microlites, globulites, etc., that appear to be first steps in crystal- 

 lization, though many of them do not take definite geometrical shapes 

 and some do not show the optical characters of crystals. There are 

 minute globules (globuhtes), needles, and hair-Hke bodies (trichites) 

 of more or less indeterminate nature, together with other forms that 

 can be seen to be certainly the initial forms of well-known minerals. 



Fig. 338.— Flow structure in rhyolite. Nearly natural size. (Photo, by Church.) 



The obsidians. — Of the compact glassy rocks, obsidian is the best type. 

 It is essentially a natural glass, formed usually of acid silicates. It has 

 the close texture, conchoidal fracture, and other qualities of glass. It is 

 usually black, but sometimes red, brown, purple, bluish, or gray. While 

 chiefly of glass, it usually contains more or less of the incipient crystals 

 above described, showing that even here the first step in the gradation 

 to the next or the crystaUine stage has been taken. These incipient 



