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cubical lamellar feltfpars, of a changeable 

 Vvhitenefs. 



When expo fed to the furnace for a few 

 hours it becomes black, and after a longer 

 time fufes into a black, compa<5t, and very 

 fmooth enamel, which fets in motion the 

 magnetic needle, though it produced nofuch 

 €fFe(5l when it was porphyry. The felt- 

 fpars remain entire. 



In this part of my circuit round Allcuda 

 J have defcribed two kinds of lavas, the 

 one found in detached globes, and the other 

 in a current ; which, however, from the 

 identity of their nature, may be confidered 

 ^s one only ; both having for their bafe the 

 petrofilex, and containing fhoerls and felt- 

 fpars : they are therefore both porphyritic. 

 And as the rock laft defcribed is a porphyry 

 with a petrofiiiceous bafe, it appears that 

 they all three derive their origin from one 

 common matrix, except that one portion of 

 it has been fubjeded to fufion, and the other 

 femained untouched. 



A little 



