^4 EXPERIMENTS on WHINSTONE and LAVA. 



Both kinds of glafs yielded highly charadlerifed cryftallltes ; that 

 laft mentioned, having its felfpars entire, produced a fubftancc 

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

 black cryflalline bafis. The cryftals formed in this bafis are 

 fo complete in one example, that they are feen proje<5ling in- 

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

 to make it fparkle all over. Thefe black cryflals feeni to be 

 hornblend of new formation. We have found, by feme late 

 experiments, that they are confiderably more refradory than 

 the cryflallite in which they lie, and are equally infufiblc with 

 fome fpecies of natural hornblend. 



No. 4. M'>6;« from the neighbourhood of Diiddingjione Loch. ' 



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

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

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

 to be greeniih-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. i.. 



No. 5. Whin of Salifhury Craig near Edinburgh. 



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

 greenifli -white matter, both in minute grains. The greenilli- 

 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. i. It has confiderable luftre, chiefly 

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

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



cryftallite. 



