Chap. 17.] Ceurje cf a River of Lava. 105 



a river with mafles of ice floating on its furface. Fluid 

 lava fometimes flows for a confiderable diftance, un- 

 der the tenacious fcum, and again appears beyond it 

 in all its fplendor. In the night time the coiirfe of 

 the lava appears like flame, in confequence of the bi- 

 tumen, which is mixed with the lava, and which Is 

 fuppofed to be the principal caufe of its fluidity. In 

 the day-time its courfe is marked by a thick white 

 fmoke. 



Mr. Kirwan divides lavas into three kinds, the cel- 

 lular, the compact, and the vitreous. All lavas arc 

 more or lefs magnetic, give fire with fteel, are of a 

 granular texture, and melt without the addition of 

 other fubftances. The cellular lavas are fuch as have 

 undergone only the firft and loweft degree of fufion, 

 being juft foftened and heated fufficiently to expel the 

 fixed air contained in the matter from "which rhey are 

 formed, which feems to be argillaceous flate -, hence 

 they abound in fmall cavities occafioned by the expan- 

 fion of that air. The fpecific gravity of fome lavas 

 from thefe cavities is fo fmall, that they float for fome 

 time on water. From this circumftance they have 

 fometimes been miftaken for pumice-flone j but they 

 differ from it, becaufe their texture is never filamen- 

 tous. The perfectly vitrified lavas muft have been 

 expofed to an immenfe heat, as they are very difficultly 

 fufed without addition. M. Sauffure has ingenioufly 

 imitated all the different kinds of lavas, by different 

 degrees of fufion of the earthy fubftances from which 

 they are formed. M. Bergman analyzed a fpecimen 

 of the perfect kind, and found an hundred parts to 

 confift of forty-nine of filiceous earth, thirty-five of 

 Argillaceous, four of calcareous earth, and twelve of 

 iron. 



The beds of lava are deepeft and narroweft near the 



crater, 



