82 GLOSSARY. 



which distinguish them from two other great series of strata, 

 the secondary and primary, which lie beneath them. See 

 Tables, p. 61, &c. 



TESTACEA. Molluscous animals, having a shelly covering. Etym., 

 testa, a shell, such as snails, whelks, oysters, &c. 



THIN OUT. When a stratum, in the course of its prolongation in 

 any direction, becomes gradually less in thickness, the two sur- 

 faces approach nearer and nearer ; and when at last they meet, 

 the stratum is said to thin out, or disappear. 



TRACHYTE. A variety of lava essentially composed of glassy fel- 

 spar, and frequently having detached crystals of felspar in 

 the base or body of the stone, giving it the structure of por- 

 phyry. It sometimes contains hornblende and augite; and 

 when these last predominate, the trachyte passes into the 

 varieties of trap called greenstone, basalt, dolorite, &c. The 

 term is derived from rpacvc, trachus, rough, because the rock 

 has a peculiar rough feel. 



TRAP and TRAPPEAN ROCKS. Volcanic rocks composed of felspar, 

 augite, and hornblende. The various proportions and state of 

 aggregation of these simple minerals, and differences in exter- 

 nal forms, give rise to varieties, which have received distinct 

 appellations, such as basalt, amygdaloid, dolorite, greenstone, 

 and others. The term is derived from trappa, a Swedish 

 word for stair, because in Sweden the rocks of this class often 

 occur in large tabular masses, i*ising one above another, like 

 the steps of a staircase. For further explanation, see vol. iii. 

 p. 359. 



TRAVERTIN. A limestone, usually hard and semi-crystalline, depo- 

 sited from the water of springs holding lime in solution. The 

 word is Italian, and a corruption of the term Tiburtinus, the 

 stone being formed in great quantity by the river Anio, at 

 Tibur, near Rome, and hence it was called by the ancients 

 Lapis Tiburtinus. 



TROPHI, of Insects. Organs which form the mouth, consisting of 

 an upper and under lip, and comprising the parts called man- 

 dibles, maxilla), and palpi. 



TUFF, or TUFO. An Italian name for a variety of volcanic rock, of 

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

 agglutination of fragments of scoriaj and loose matter ejected 

 from a volcano. 



TUFACEOUS. A rock with the texture of tuff or tufo, which see. 



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

 Etym.) turbinatus, made like a top. 



