Argillaceous Genus. 8 1 



that has fuffered a moderate heat. Its harden- 

 ing power arifes from the dry flate of the 

 half-baked argillaceous particles, which makes 

 them imbibe water very rapidly, and thus ac- 

 celerates the defication of the calcareous 

 part ; and alfo from the quantity and lemi- 

 phlogifticated ftate of the iron contained in 

 it. It is found not only in Italy but alfo in 

 France, in the provinces of Auvergne and 

 Limoges, and alfo in England, and elfewhere* 



Traqfsy or Terras, 



This differs but little in its principles front 

 pouzzolana, but is much more compact and 

 harder, porous and fpungy. It is generally 

 of a whitim yellow colour, and contains more 

 heterogenous particles, as fpar, quartz* fhoerl, 

 &c. and fomething more of calcareous Earth : 

 it effervefces with acids, fs magnetic, and fu- 

 fible perfe. When pulverized, it ferves as a 

 cement, like pouzzolana. It is found in Ger- 

 many and Sweden. 



Tufa. 



Volcanic aflies concreted with various other 

 fpecies of (lone, but in which argill predo- 

 minates, forms the ftone thus called : it is 

 harder than traafs, but ftill porous and 

 fpungy. 



G White 



