icS tferra Puzzolana. [Book VI. 



of a fimilar figure. On the whole, Mr. Kirwan thinks 

 it much more probable that bafaltes furnifh the mate^ 

 rials of volcanic products, than that they themfelves are 

 of volcanic origin. 



IV. TERRA PUZZOLANA or Terras, is a volcanic 

 production, of a grey, brown, yellowifh or blackilh 

 colour, loofe, granular, or dufly and rough, porous 

 and fpongy, refembling a clay hardened in the fire, 

 and then reduced to a grofs powder. Its mod diftin- 

 guifhing property is, that when mixed with about one- 

 third of its weigHt of lime and water, it hardens very 

 fuddenly, and forms a cement which is more durable 

 in water than any other. Its indurating power fcems 

 to arife from the dry ftate of the half-baked argilla- 

 ceous particles, which caafes them to imbibe water 

 very rapidly, and thus the deficcation of the calcareous 

 earth is accelerated. It is found not only in Italy, but 

 alfo in France, in the provinces of Auvergne and Li- 

 moges, and alfo in England ancl elfewhere. Accord- 

 ing to Bergman's analyfis, one hundred parts contain 

 from fifty five to fixty of filiceous earth, nineteen or 

 twenty of argillaceous, five or fix of calcareous, and 

 from fifteen to twenty of iron. 



