6o EXPERIMENTS on IVHINSTONE atid LAVA. 



refunics its flony character. But, not content with analogy- 

 alone, I refolved^to afcertain the truth of thefe conclufions in a 

 direcl manner, and performed the following experiments with 

 fpecimens of fix different lavas, four of which, to my certain, 

 knowledge, had made part of external volcanic currents. In the 

 prefent ftate of geology, too much pains cannot be beftowed in 

 afcertaining that the fpecimens colledted are really lavas, fince 

 tliis circumflance has been frequently overlooked, as I fliall en- 

 deavour to fliew, when I fpeak of the differences between them 

 and whinftone. 



No. I. Lava of Catania. 



This is the celebrated lava, which, in 1669, laid wafle great- 

 part of the town of Catania. The interior part of the cur- 

 rent, (accurately defcribed by M. Dolomieu, IJles Ponces, 

 p. 256.*), from which the fubjedl of our experiment was taken, 

 confifts of a light grey bafis, interfperfed with cryftals of fel- 

 fpar and of fchorl, (augit). It bears a general i-efemblance to 

 the rock of the bafaltic columns on Arthur's Seat, and exhibited 

 the fame phenomena in our experiments. After ftrong heat, the 

 whole was reduced, by rapid cooling, to pure black glafs ; but 

 when the heat applied was moderate, the felfpars remained 

 unchanged. Being maintained, after a fecond fufion, in a 

 temperature of 28, both thefe glaffes yielded ftony and cryftal- 

 lized fubftances, fomcwhat lefs fufible than the original ; and 



when 



* " Elle eft formee d'une pate de roche de coine grife, a grains fins, melee 

 " d'ecailles, et de criftaux de feld-fpath de meme couleur ; e]le contient un tres 

 " grand ronnbre de criftaux de fchorl noir, et de grains de cryfolites jaunes, les uns 

 " et les autres quelquefois chatoyans, de difFerentes ccmleurs dans leiirs fraftures. — 



" ^Cette lave a une calTure feche, et un grain rude, fiirtout dans 



" le centre des courans ; c'eft la ou elle a toujours conferve une couleur plus claire, 

 " qai doit etre celle de fa bafe ; fur les bords es les furfaces elle s'eft fort noircie ; 

 " elle y a acquis une aflez forte aftion fur I'aiguille aimantee que celle du centre n'a 

 " prefque point," 



