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thefe prodigious malTes of rocks were 

 igneous eruptions, or, in fad:, currents of 

 lava, of which the porofity of fome of 

 them was a very evident and fatisfaflory 

 proof. It is indubitable that the elaftic 

 gafe?, which generate pores and vacuities 

 m the recks acfled on by lire, cannot pro- 

 duce that efTevfl unlefs thefe rocks are foft. 

 ened fufFiciently to yield to the expanfive 

 force of thefe aeriform fubftances, which 

 muPc precifely happen in their a6lual lique- 

 faction. This, though fufficiently evident 

 in iifelf, is undeniably proved by experi- 

 ments I made in the furnace. When the 

 porphyries with a petrofiliceous or horn- 

 fione bafe, and, generally, when compact 

 Javas alTuai^d the fimple vitreous varnifh, 

 and fomewhat of a vitreous afpe£l inter- 

 nally, but v/ithout the pieces conglutinating 

 and forming one fmgle uniform mafs, they 

 rarely or never contained bubbles ; w^hich, 

 on the contrary, were confpicuous and nu- 

 merous when the fufion w^as complete, and 

 iht larger, the greater the degree of liqui- 

 dity produced by the iire in the fljony bodies. 



The 



