VOLCANIC ROCKS. 599 



tion of these rocks shall have been formed, it will 

 probably not be difficult to construct a new cata- 

 logue, by adopting a different division with res- 

 pect to the essential and unessential minerals. 



SYNOPSIS OF THE VOLCANIC ROCKS. 



FIRST DIVISION. 

 More or less perfectly vitreous. 



FIRST SUBDIVISION. 



Solid. 



A/ Obsidian: volcanic glass. 



a. Massive. 



b. With an imperfect laminar structure, 



indicated by a striped appearance, 

 and by the action of the weather. 



c. Concretionary, imperfectly spheroidal 



or granular. 



d. Fibrous, loose, resembling spun glass, 



rare. 



e. Porphyritic; enclosing felspar. Ob- 



sidian porphyry. 



The most common colours are black, dark 



