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pltcK-ftone lavas and volcanic glafs. The 

 idea of volcanic glafs is derived from that 

 we have from our common glafles j the 

 infeparable chara6:ers of which, as we well 

 know, are, that they fhould have a fhining 

 and bright furface ; be compofed of indif- 

 cernible particles ; break into angular pieces, 

 extremely thin at the edges, and with acute 

 and piercing points ; with a fradure either 

 entirely ffnooth and clear, or, as we fre- 

 quently find, with waving and curved 

 flreaks ; not to mention its tranfparency, a 

 quality known to every one, and, in glafs,one 

 of the moft valuable. The fame, precifely, 

 are the charaders of the greater part of 

 volcanic glafles ; but the defcription of the 

 pitch-done lavas is very different. They 

 have commonly a dead and dull iuftre ; 

 their pafte is fine, indeed, but not compa- 

 rable in finenefs to that of glafs ; they are 

 lefs fmooth in the fradures, lefs fharp and 

 cutting at the edges, and have little or no 

 tranfparency. They are likexvife diftin- 

 guiflied by another property from volcanic 

 glafles, the greater part of which are fuifi- 



ciently 



