( l<?4 ) 

 what fibrous : in line, they perfedly refem»« 

 ble in their ftrudture the fhoerls incorporated 

 in lavas. 



The tufa, after remaining two or three 

 hours in the furnace, aflumes a red colour, 

 and becomes hard ; its raagneiifm is Uke-r 

 wife ftrong, though before it was fcarcely 

 perceptible. A longer continuance in the 

 fame fire reduces it to a porous fcoria, which 

 does not lofe its magnetifm, and the black 

 colour it acquires renders more confpicuous 

 a number of white feltfpars which before 

 were not difcernible in the tufa. The 

 fhoerls are femi-vitrified, and aflume a 

 yellowifh tinge, 



In the remainder of my circuit round the 

 ifland, till I returned to the place whence I 

 fet out, I obferved no other interefting ob-^ 

 jed:s, excepting a long trad; of prifmatic 

 lavas, fimilar to thofe I have already de- 

 fcribed. 



I fliall conclude what I have to fay on 



thefe 



