Ch. XXVIIL] of volcanic rocks. 607 



canic products seems to imply that it belongs properly to the metamorphic 

 class ; and, even when it is found in dikes cutting through aqueous forma- 

 tions, it may be an altered basalt, which abounded greatly in olivine. 



Tephrine, synonymous with lava. Name proposed by Ales. Brongniart. 



Toadstone. A local name in Derbyshire for a kind of wacke, which see. 



Trachyte. Chiefly composed of glassy felspar, with crystals of glassy felspar. 

 See?. 599." 



Trap Tuff. See p. 602. 



Trass. A kind of tuff or mud poured out by lake-craters during eruptions ; com- 

 mon in the Eifel, in Germany. 



Tuff. Syn. Trap tuff, volcanic tuff. See p. 602. 



Vitreous Lava. See Pitchstone and Obsidian. 

 Volcanic Tuff. See p. 602. 



Wacke. A soft and earthy variety of trap, having an argillaceous aspect. It re- 

 sembles indurated clay, and when scratched, exhibits a shining streak. 

 Whinsjone. A Scotch provincial term for greenstone and other hard trap rocks. 



