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Travertin. A concretionary limestone, usually hard and semi- 

 crystalline, deposited from the water of springs holding lime 

 in solution. Elym. This stone was called by the ancients 

 Lapis Tiburtinus, the stone being formed in great quantity by 

 the river Anio, at Tibur, near Rome. Some suppose travertin 

 to be an abbreviation of trasteverino from trans-tiburtinus. 



Tuff or Tufa. An Italian name for a variety of volcanic rock of 

 an earthy texture, seldom very compact, and composed of an 

 agglutination of fragments of scoria;, and loose matter ejected 

 from a volcano. 



Turbinated. Shells which have a spiral or screw-form structure. 

 Etym., turbinatus, made like a top. 



Unconformable. See Conformable. 



Veins, Mineral. Cracks in rocks filled up by substances different 

 from the rock, which may either be earthy or metallic. Veins 

 are sometimes many yards wide; and they ramify or branch 

 off into innumerable smaller parts, often as slender as threads, 

 like the veins in an animal, and hence their name. 



Wacke. A rock nearly allied to basalt, of which it may be regarded 

 as a soft and earthy variety. 



Zeolite. A family of simple minerals, including stilbite, mcsotype, 

 analcime, and some others, usually found in the trap or volcanic 

 rocks. Some of the most common varieties swell or boil up 

 when exposed to the blow-pipe, and hence the name £sw, zeo, 

 to boil, and Xi0os, lithos, stone. 



Zoophytes. Corals, sponges, and other aquatic animals allied to 

 them, so called because, while they are the habitation of animals, 

 they are fixed to the ground, and have the forms of plants. 

 Etym., £wov, zoon, animal, and tporov, phyton, plant. 



