id form of the same deposit is called "travertin," into
which it passes.

TUFA, VOLCANIC. See "Tuff."

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

TUFF, or TUFA VOLCANIC. An Italian name for a variety of volcanic
rock of an earthy texture, seldom very compact, and composed of an
agglutination of fragments of scoriae and loose materials ejected
from a volcano.

TURBINATED. Shells which have a spiral or screw-form structure.
_Etym._, _turbinatus_, made like a top.

TURRILITE. An extinct genus of chambered shells, allied to the
Ammonites, having the siphuncle near the dorsal margin.

UNCONFORMABLE. See "Conformable."

UNOXIDIZED, UNOXIDATED. Not combined with oxygen.

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, hence their name.

VERTEBRATED ANIMALS. A great division of the animal kingdom,
including all those which are furnished with a back-bone, as the
mammalia, birds, reptiles, and fishes. The separate joints of the
back-bone are called _vertebrae_, from the Latin verb _verto_, to
turn.

VESICLE. A small, circular, inclosed space, like a little bladder.
_Etym._, diminutive of _vesica_, Latin for a bladder.

VITRIFICATION. The conversion of a body into glass by heat.

VOLCANIC BOMBS. Volcanoes throw out sometimes detached masses of
melted lava, which, as they fall, assume rounded forms (like
bomb-shells), and are often elongated into a pear-shape.

VOLCANIC FOCI. The subterranean centres of action in volcanoes,
where the heat is supposed to be in the highest degree of energy.

WACKE. A rock nearly allied to basalt, of which it may be regarded
as a soft and earthy variety.

WARP. The deposit of muddy waters, artificially introduced into low
lands. See p. 326.

ZEOLITE. A family of simple minerals, including stilbite, mesotype,
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 of [Greek: zeo], _zeo_,
to boil, and [Greek: lithos], _lithos_, stone.

ZOOPHITES. Corals, sponges, and other aquatic animal