510 DOMAIN v. CALCAREOUS. 



moss, grass, and other vegetables. The forma- 

 tion in many instances is so rapid, that it is ap- 

 vr y modern, plied to the purposes of art. Nests of birds, 

 and other small objects, are subjected to the 

 stream ; and, when covered by the deposition, 

 are said to be petrified ; an erroneous idea, for 

 they are merely clothed with tufa. Such is the 

 tufa common on the banks of the Tees, and 

 other rivers in the north of England. By its 

 lightness it is well calculated for vaults and roofs 

 in buildings, where the use of wood would be 

 dangerous; and, by its open intervals, admits 

 the mortar, so as to form as it were one coherent 

 mass ; and it was used by the ancients in many 

 constructions. The Pharos, at Dover, is chiefly 

 built of tufa, from the north of England*. 



But as the Italians first used the word tufo, 

 and seem more generally to have applied it to 

 volcanic accretions, there is no impropriety in 

 extending it, as is often done, to many loose 

 and porous stones, evidently of recent forma- 

 Conchitic. tion. Thus the shelly tufa of Gmelin, consist- 

 ing of broken fragments of shells, with sand and 

 gravel, loosely joined by a calcareous cement, 

 might perhaps be more properly classed here 



* This tufa seeming to join substances together, was exhibited by 

 the surgeons in. the fracture of boaes, and called osteecolla. 



