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caiife why certain lavas, differing from in- 

 numerable others, become bafahiform; fmce, 

 if this configuration depended on conge- 

 lation, it muft be found in all lavas when 

 they had ceafed to flow. The firft writer, 

 to my knowledge, who has adverted to 

 this is M. de Luc, who, in the fecond vo- 

 lume of his Travels, is of opinion that they 

 have taken this regular figure in the fea, 

 by the fudden condenfation which took 

 place on their flowing into it in a liquid 

 fl;ate ; other fecondary circumflances, how- 

 ever, concurring, fuch as a greater homo- 

 geneity, and a certain attradion of their 

 parts. 



Of the fime opinion is M. Dolomieu ; 

 though he does not deny that even porous 

 lavas may fometimes, likewife, take the 

 form of prifns. The former of thefe opi- 

 nions is little lefs than hypothetic, while 

 the latter is fupported by fads too import-^ 

 ant to be curibrily fl:ated. M. Dolomieu 

 obferves that all the currents of the lavas of 

 Etna, the periods of which are preferved in 



hi (lory, 



