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from his own obfervation that thefc 

 matters have really fuffered the action of 

 fire, if he wifhes to convince the philofo- 

 phical world he muft fpecify their nature ; 

 as whether they are puzzolanas, afhe?, 

 glafles, enamels, fcorix or lavas. The 

 precifion and exadnefs required in the pre- 

 fent difcerning age likewife render it necef- 

 fary that he fhould charaderize thefe dif- 

 ferent volcanized fubftances ; and it will be 

 to little purpofe that he adduces in proof of 

 an extinguifhed volcano the remains of a 

 current of lava, unlefs he properly defcribes 

 it, fuice the advancement of this fcience 

 depends entirely on exad defcrlptions of 

 produds affeded by the fire. The moft 

 able volcanifts of the prefent time write 

 after the manner of accurate mineralogies. 

 Among thefe volcanifts, the firft place is 

 certainly due to M. Faujas de Saint Fond, 

 as is fufficiencly proved by his Mineralogy 

 af Volcanos<f 



And here I cannot refrain from making 

 fome obfervations relative to lavas in gene- 

 ral. 



