( 34 ) 



The third only differs from the feconcJ 

 by being fomewhat more compaift, harder, 

 and containing fewer feltfpars. 



The fourth, which in folidity, weight, 

 and hardnefs, exceeds the three preceding, 

 has a black ferrugineous colotrc, an earthy 

 -fradure, adheres llightly to the tongue, and 

 emits the ufual argillaceous odour. It 

 moves the magnetic needle at the diftance 

 of half a line. 



AH thefc four kinds of lavas are change<J, 

 in the furnace, into vitreous fcorise, but with- 

 out the fufion of their feltfpars. 



Having mentioned thefe, it will be necef- 

 fary to defcribe, fomewhat more at length, 

 another fpecies of lava, which is enrich- 

 ed with a great number of extremely beau- 

 tiful volcanic chryfolites. 



This lava has for its bafe a foft horn-ftone : 

 it is of a dark brown colour, and unequal 

 in its fractures on account of the fiffures 



6 which 



