Principles of Geology Part 102

STUFAS. Jets of steam issuing from fissures in volcanic regions at a temperature often above the boiling point.

SUBAPENNINES. Low hills which skirt or lie at the foot of the great chain of the Apennines in Italy. The term. Subapennine is applied geologically to a series of strata of the Older Pliocene Period.

SYENITE. A kind of granite; so called, because it was brought from Syene in Egypt.

TALUS. When fragments are broken off by the action of the weather from the face of a steep rock, as they acc.u.mulate at its foot, they form a sloping heap, called a talus. The term is borrowed from the language of fortification, where _talus_ means the outside of a wall of which the thickness is diminished by degrees, as it rises in height, to make it the firmer.

TARSI. The feet in insects, which are articulated, and formed of five or a less number of joints.

TERTIARY STRATA. A series of sedimentary rocks, with characters which distinguish them from two other great series of strata--the secondary and primary--which lie _beneath_ them.

TESTACEA. Molluscous animals, having a sh.e.l.ly covering. _Etym._, _testa_, a sh.e.l.l, such as snails, whelks, oysters, &c.

THALLOGENS. A cla.s.s of flowerless plants including all those that have no defined axis, stem, or leaves; as Lichens, Seaweeds, and Fungi. _Etym._, ?a????, _thallos_, a branch, and ?e?es??, _genesis_, increase.

THERMAL. Hot. _Etym._, ?e???, _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 gla.s.sy 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 pa.s.ses into the varieties of trap, called Greenstone, Basalt, Dolorite, &c. The term is derived from t?a???, _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 cla.s.s sometimes occur in large tabular ma.s.ses, 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 quant.i.ty 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 polis.h.i.+ng 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 solid form of the same deposit is called "travertin," into which it pa.s.ses.

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. Sh.e.l.ls which have a spiral or screw-form structure.

_Etym._, _turbinatus_, made like a top.

TURRILITE. An extinct genus of chambered sh.e.l.ls, 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 s.p.a.ce, like a little bladder.

_Etym._, diminutive of _vesica_, Latin for a bladder.

VITRIFICATION. The conversion of a body into gla.s.s by heat.

VOLCANIC BOMBS. Volcanoes throw out sometimes detached ma.s.ses of melted lava, which, as they fall, a.s.sume rounded forms (like bomb-sh.e.l.ls), 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, a.n.a.lcime, 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 ?e?, _zeo_, to boil, and ?????, _lithos_, stone.

ZOOPHITES. 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 form of plants. _Etym._, ????, _zoon_, animal, and f?t??, _phyton_, plant.