378 DOMAIN XII. VOLCANIC. 



NOME IV. TUFO. 



Composition. This may be regarded as the fourth and last 

 of the great volcanic ejections. It is chiefly 

 composed of volcanic sand and powders, or 

 what are absurdly called ashes, of pulverised 

 lava, dross, and pumice. When it consists of 

 ferruginous clay it is properly called puzzolana _,- 

 when of pumice in a recent state, rapillo or 

 lapillo. For as earths are no longer distinguish- 

 ed from stones, the difference of cohesion not 

 altering the nature of the substance, so tufo* 

 may be regarded as of various indurations. 

 These remarks, however, naturally lead to two 

 grand divisions ; the HARD tufo, which is used 

 as stone; and the SOFT, or incoherent tufo, 

 which is also called puzzolana, tarras, &c. 

 Tufo of Troil has observed, that the greater part of the 



Iceland. 



Icelandic mountains consist of tufo; and Hecla 

 often ejects brown and black pumice, with sand 

 and powder, of which substances it chiefly con- 

 sists, interspersed with fragments of slate, either 

 originally red or changed by fire. Perhaps the 



* Italian writers always put tufo. It might be a not unuseful 

 distinction, as already stated, to confine tufa to the calcareous and 

 other depositions merely aquatic. 



