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 rpcucvQ, 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, rising 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, maxillae, 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 scorise 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., turbinates, made like a top. 
