424 DOMAIN XII. VOLCANIC. 



creases in bulk, Mem. Par. 1781, p. 277: and so 

 does puzzolana when formed into mortar, Higgins 

 on Cements, 125. One principal use of lime 

 seems to be to heat the water, as while cold it 

 cannot readily pervade the caked argil that invests 

 the ferruginous particles ; yet, in time, even cold 

 water may pervade it, and produce hardness ; and 

 hence lavas become harder when moistened, as 

 M. Dolomieu has observed, Ponces, 417. If 

 the mortar be long exposed to the atmosphere, 

 fixed air, as well t as pure air, will unite to the iron, 

 rust will be produced, and the mortar will not 

 then harden, as Dr, Higgins has also noticed. 

 Clay, over which lava has flowed, is frequently 

 converted into puzzolana, Ponces, 332. But vol- 

 canic scoriae never afford it ; ibid ; either because 

 they are much calcined, or retain sulphur, or its 

 acid." 



" TRASS OR TARRAS. 



" I couple this with puzzolana, on account of 

 their similarity to each other, and not because I 

 look upon it as constantly, and necessarily, a vol- 

 canic production. On the contrary, I believe it 

 to be generally the product of pseudo- volcanoes, 

 or external fires. 



"It is found in many places, but principally 

 near Andernach, in the vicinity of the Rhine ; also 



