54 EXPERIMENTS on WH1NST0NE and LAVA. 



Both kinds of glafs yielded highly characterifed cryflallites ; that 

 laft mentioned, having its felfpars entire, produced a fubftance 

 like porphyry, in which the white felfpars were embedded in a 

 black cryftalline bafis. The cryftals formed in this bafis are 

 fo complete in one example, that they are feen projecting in- 

 to the cavities, and {landing erect on the external furface, fo as 

 to make it fparkle all over. Thefe black cryftals feem to be 

 hornblend of new formation. We have found, bv fome late 

 experiments, that they are confiderably more refractory than 

 the cryftallite in which they lie, and are equally infuuble with 

 fome fpecies of natural hornblend. 



No. 4. Whin from the neighbourhood of Duddingficne Lo 



It has for its bafis a black bafalt of an uneven fracture. 

 In it are embedded augit in numerous cryftals, felfpar in a fmall- 

 er proportion, and difperfed grains of olivin. The felfpar feems 

 to be greenim-white, with confiderable luftre and tranfparency. 

 The ftone gives fire with fteel, and has a flight earthy fmell 

 when breathed on. Its glafs yields a fine grained cryftallite, 

 like that of No. 1. 



No. 5. Whin of Salijbury Craig near Edinburgh. 



This fpecies is an aggregate of black hornblend, and of a 

 greenilh-- white matter, both in minute grains. The greeiiifh- 

 white matter refembles felfpar, but is much more fufible. The 

 general characters are nearly the fame with thofe of the fpeci- 

 mcn already defcribed, No. 1. It has considerable luftre, chiefly 

 from the hornblend ; an earthy fmell when breathed on ; and 

 gives fome fparks with fteel. Its glafs yielded a highly facetted 



cryftallite, 



