eek:
genesis], _genesis_, increase.

THERMAL. Hot. _Etym._, [Greek: thermos], _thermos_, hot.

THERMO-ELECTRICITY. Electricity developed by heat.

THIN OUT. When a stratum, in the course of its prolongation in any
direction, becomes gradually less in thickness, the two surfaces
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 felspar,
and frequently having detached crystals of felspar in the base or
body of the stone, giving it the structure of porphyry. 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 [Greek: trachus],
_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 external 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 the rocks
of this class sometimes occur in large tabular masses, rising one
above another like steps.

TRAVERTIN. A white concretionary limestone, usually hard and
semi-crystalline, deposited from the water of springs holding lime
in solution.--_Etym._ 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 trasterverino from transtiburtinus.

TRIPOLI. The name of a powder used for polishing metals and stones,
first imported from Tripoli, which, as well as a certain kind of
siliceous stone of the same name, has been lately found to be
composed of the flinty cases of Infusoria.

TROPHI, of Insects. Organs which form the mouth, consisting of an
upper and under lip, and comprising the parts called mandibles,
maxillae, and palpi.

TUFA, CALCAREOUS. A porous rock deposited by calcareous waters on
their exposure to the air, and usually containing portions of plants
and other organic substances incrusted with carbonate of lime. The
more sol