d'Or. 



the lake cf An- 

 dernach 



J 82 ON THE CRYSTALS IN LAVAS, 



difference accounted for much more naturally by the ab-- 

 sence of leucite in those strata, from which the lavas of 

 Etna proceeded ? 

 Lavas of jHe- The same variations are observed in the lavas of different 

 volcanoes. Those of Hecla, of which I have considerable 

 specimens brought home by Sir Joseph Ba iki, contain 

 neither pyroxene schoerls, nor leucites, nor chrysolites, but 

 a great many small, white, cracked, crystalline substances, 

 from the size of a grain of hempseed to that of a pea, of 

 an irregular figure^ having the appearance and hardness of 

 and Mont- quartz, of which they appear to be fragments. The lavas 

 of Mont-d'Or, an ancient volcano of Auvergne, contain large 

 crystals of amphibole, or hornblende, and feldspar, whit^ 

 show by their cracks and vitreous reflections, that they 

 have experienced the action of incandescent lava; and.ia 

 other ancient volcanoes in Auvergne we find pyroxene 

 schoerls without leucites. 

 Leucites in The smaU gravel of the volcanic lake of Andernach is 



filled with loose pyroxene schoerls, whole and in pieces. 

 Should we find in this state the confused radii of coole4 

 glass, which formed part of the mass, and couid oaly be 

 separated from it in shaoeless fragments? 

 Eruption of Among the facts I adduced against the opinion of Sir 



Vesuvius ia James Hall, quoted by Mr. Fl. de Bellevue, whob.e opinioin 

 is the same, I mentioned a singular eruption of Vesuvius, 

 that happened in 1754. A mouth opened nearly at the le- 

 vel of the valley, which separates the present cone from 

 Grotto lined Mount Somma. At the rise of the lava this mouth formed 

 with the stalac- q^ o;rotto, which it lined by its spirtings with a quantity of 

 titical scoriie ^. .' ^, .^. , „ "^ ^u ■ ^\ • j-^ c i-i 



sconce m a stalactitical form, the mtertwmed jets ot which 



are from three to sjx lines in diameter, of a reddish colour,, 



containing py- and full of blebs. In the fragments of these jets I have 



Toxenes. found pyroxene schoerls in a state of perfect crystallization, 



and with their deep olive colour. These spirtings indicate, 



that the lava v/as in a high degree of fusion, and such slen^ 



der jets must have cooled and hardened the moment they 



were separated. 



No slow cool- We have here no slow refrigeration to form crystals, nor 



••us hcje. mass sufficient to give rise to crystalline forms by this 



mean ; yet we ftnd m these jets pyr*oxene schoerls, and for 



the 



