( 24^ ) 



depriving it of its tranrparency, and render-* 

 ing it full of cracks, and extremely friable ; 

 all which changes follow on expofmg it a 

 fliort time to the furnace. I fhall obferve, 

 further, that two of thefe nodofities being 

 left in a crucible placed on burning charcoal 

 only for a quarter of an hour, loft tbelr 

 amethyftine colour, and acquired a more 

 than fuperficial whitenefs, with a number of 

 cracks, and very apparent friability. I, 

 therefore, cannot believe that they have been 

 taken up by the way, and enveloped in the 

 lava when it flowed, as fometimes happens, 

 V7hen nerv torrents meet with extraneous 

 ftones in their paffage. I am, in faifl, of 

 opinion, thatthefe fmalimafTes of quartz have 

 been produced pofterior to the congelation of 

 the lava, by the filtration of water impreg- 

 nated with the molecules of quartz, which 

 has penetrated into fome fmali cavities, and 

 gradually filled them ; in the fame manner 

 as we have explained the formation of the 

 calcedonic globes found in fome of the lavas 

 of Lipari *. 



* See Chap. XV. 



The 



