( 41 ) 



bafe has the red colour of brick. It is 

 found in detached mafTes, fome of which 

 weigh feveral thoufand pounds. It is corn- 

 pad:, and fcaly in the fractures. The pieces 

 broken from it are irregular ; the tlitnnell 

 are tranfparent at the edges ; and they give 

 fparks moderately with fteel. The colour 

 of the bafe has given the feltfpars a reddiih 

 tinge, as we fee in certain oriental porphy- 

 ries. 



But has this porphyry fufFered fufion, or 

 is it in its natural ftate, and, at mofl, cal- 

 cined when it was ejeded by the volcano ? 

 I cannot pretend pofitively to decide ; but I 

 incline to the latter opinion more than to the 

 former, fmce an alteration is vifible, even in 

 the internal parts, which appears to be the 

 effedt of a true calcination. 



In the furnace, the fubftance of this rock 

 becomes foft, but does not fufe : the felt- 

 fpars remain unchanged, but the fhocrls are 

 vitrified. 



The 



