480 [Assembly 
Tufa, calcareous and silicious. Porous or earthy deposits from springs 
of water, containing lime or silex. 
Tuff or Tufa, Italian. A name for a volcanic rock of an earthy tex- 
ture. 
Unconformable. See Conformable. 
Veins. Openings or fissures in rocks, filled with stony or metallic mat- 
ter. 
Zoophites. Animals of the lowest order, as sponges, corals, &c. 
