^8 Experiments on Whinjlune and Lava, 



fcoria, the fummit and fides of which are covered by the ftony mafs, fo that no crater is 

 vifible. It ftruck me on feeing it, and I found M. Dolomieu had formed the fame opi- 

 nion, that the lava had rifen up in a perpendicular direQion, and had flowed over on all 

 fides. Its great thickiiefs, and fmall extent, feem to favour a conje£i:ure which this 

 naturalift has formed with regard to feveral lavas, that they were erupted at the bottom of 

 an ocean which once covered Sicily, and, being quickly cooled by the conta£l of water, 

 had been prevented from flowing far. The conjedlure feems plaufible enough * ; and, 

 having no proof that this fubfl:ance made part of an external current, as I have with 

 refpedt to the firft two mentioned, I do not exhibit it as a lava with the fame confidence. 

 "Whatever be its hiftory, however, it poflefl"es the chemical properties common to whin and 

 lavas. 



Its glafs yielded a dark grey cryftallite of uniform texture. Befide it in the drawer, 

 HOW on the table, I have placed a cryftallite, formed from the whin No. i . which refembles 

 it in every refpeft; 



No. 4. Lava of Iceland,. 



I received the fpecimen from a perfon who found it on the fpot ; but not being ac- 

 quainted with the circumftances of its original pofition, I cannot be certain that it is a lava. 

 It has however every appearance of being fuch. 



It is a blue homogeneous fubftance, having fome chryfolites fcattered irregularly through 

 it. Nearly half its bulk is occupied by large air holes, which do not appear to have con- 

 tained aay extraneous matter. 



It produced a very fufible glafs, from which was formed a cryftallite much more re- 

 fa£tory than the original. 



No. y. Lava of Torre del Greco, 



This lava, which flowed from Vefuvius to the fea in the middle ages, has been an obje£b 

 of much attention, on account of its confpicuous bafaltic form. It confifts of a grey bafis 

 the frafture of which is coarfe and rough, and in which are embedded large and well 

 charadterized cryftals of fchorl, (augit), with a few chryfolites, (olivins). 



It was found to be lefs fufible than any of the others, yet its glafs cryftallized in a lower 

 temperature.. 



No. 6. Lava of Vefuviusy eruption 1785- 



From the circumftances in which the above five lavas have been feen to cryftallize after 

 fufion, it can fcarcely be doubted that the fame procefs takes place in a volcanic ftream, 

 which in confequence of its bulk, muft cool with confiderable flownefs, and that a vitreous 

 charafter would be afliiimed by the whole mafs, were it cooled with fufficient rapidity. 



• M. Dolomieu afcribes the formation of part of Mount .ffitna itfelf to a fimilar caufe. I <hall have oc- 

 <afion, in another part of this paper, to confjder that ogiuion. 



4, Tb^ 



