Principles of Geology Part 96

They are very common, especially in volcanic countries; and sometimes contain so much gas, that if a little sugar be thrown into the water it effervesces like soda-water.

CARBONIC ACID GAS. A natural gas which often issues from the ground, especially in volcanic countries. _Etym._, _carbo_, coal; because the gas is obtained by the slow burning of charcoal.

CARBONIFEROUS. A term usually applied, in a technical sense, to an ancient group of secondary strata; but any bed containing coal may be said to be carboniferous. _Etym._, _carbo_, coal, and _fero_, to bear.

CATACLYSM. A deluge. _Etym._, ?ata?????, _catacluzo_, to deluge.

CEPHALOPODA. A cla.s.s of molluscous animals, having their organs of motion arranged round their head. _Etym._, ?efa??, _cephale_, head, and p?da, _poda_, feet.

CETACEA. An order of vertebrated mammiferous animals inhabiting the sea. The whale, dolphin, and narwal are examples. _Etym._, _cete_, whale.

CHALCEDONY. A siliceous simple mineral, uncrystallized. Agates are partly composed of chalcedony.

CHALK. A white earthy limestone, the uppermost of the secondary series of strata.

CHERT. A siliceous mineral, nearly allied to chalcedony and flint, but less h.o.m.ogeneous and simple in texture. A gradual pa.s.sage from chert to limestone is not uncommon.

CHLORITIC SAND. Sand colored green by an admixture of the simple mineral chlorite. _Etym._, ??????, _chlorus_, green.

CLEAVAGE. Certain rocks, usually called Slate-rocks, may be cleaved into an indefinite number of thin laminae which are parallel to each other, but which are generally not parallel to the planes of the true strata or layers of deposition. The planes of cleavage, therefore, are distinguishable from those of stratification.

CLINKSTONE, called also phonolite, a felspathic rock of the trap family, usually fissile. It is sonorous when struck with a hammer, whence its name.

COAL FORMATION. This term is generally understood to mean the same as the Coal Measures, or Carboniferous group.

COLEOPTERA. An order of insects (Beetles) which have four wings, the upper pair being crustaceous and forming a shield. _Etym._, coleos, _coleos_, a sheath, and pte???, _pteron_, a wing.

CONFORMABLE. When the planes of one set of strata are generally parallel to those of another set which are in contact, they are said to be conformable. Thus the set _a_, _b_, Fig. 98, rest conformably on the inferior set _c_, _d_; but _c_, _d_ rest unconformably on E.

[Ill.u.s.tration: Fig. 98.]

CONGENERS. Species which belong to the same genus.

CONGLOMERATE, or PUDDINGSTONE. Rounded water-worn fragments of rock or pebbles, cemented together by another mineral substance, which may be of a siliceous, calcareous, or argillaceous nature. _Etym._, _con_, together, _glomero_, to heap.

CONIFERae. An order of plants, all of which have disks in their wood fibres, by which they are recognized in a fossil state. Their ovules are naked (see GYMNOGENS). Most of the northern kinds bear the seeds in cones; but the yew does not, nor do a host of tropical and south temperate species. _Etym._, _conus_, a cone, and _fero_, to bear.

COSMOGONY, COSMOLOGY. Words synonymous in meaning, applied to speculations respecting the first origin or mode of creation of the earth. _Etym._, ??s??, _kosmos_, the world, and ????, _gonee_, generation, or ?????, _logos_, discourse.

CRAG. A provincial name in Norfolk and Suffolk for certain tertiary deposits usually composed of sand with sh.e.l.ls, belonging to the Older Pliocene period.

CRATER. The circular cavity at the summit of a volcano, from which the volcanic matter is ejected. _Etym._, _crater_, a great cup or bowl.

CRETACEOUS. Belonging to chalk. _Etym._, _creta_, chalk.

CROP OUT. A miner's or mineral surveyor's term, to express the rising up or exposure at the surface of a stratum or series of strata.

CRUST OF THE EARTH. See "Earth's crust."

CRUSTACEOUS. Animals having a sh.e.l.ly coating or crust which they cast periodically. Crabs, shrimps, and lobsters are examples.

CRYPTOGAMIC. As.e.xual, flowerless, or Acotyledonous plants; a term applied to half the vegetable kingdom in contradistinction to Phaenogamic, s.e.xual, or flowering plants. It includes Fungi, Sea-weeds, Lichens, Mosses, Ferns, &c., which have no obvious flowers, and no cotyledons (seed-lobes) to their spores or seeds.

_Etym._, ???pt??, _cruptos_, concealed, and ?a??, _gamos_, marriage.

CRYSTALS. Simple minerals are frequently found in regular forms, with facets like the drops of cut gla.s.s of chandeliers. Quartz being often met with in rocks in such forms, and beautifully transparent like ice, was called _rock-crystal_, ???sta????, crystallos, being Greek for ice. Hence the regular _forms_ of other minerals are called crystals, whether they be clear or opake.

CRYSTALLIZED. A mineral which is found in regular forms or crystals is said to be crystallized.

CRYSTALLINE. The internal texture which regular crystals exhibit when broken, or a confused a.s.semblage of ill-defined crystals.

Loaf-sugar and statuary-marble have a _crystalline_ texture.

Sugar-candy and calcareous spar are crystallized.

CUPRIFEROUS. Copper-bearing. _Etym._, _cuprum_, copper, and _fero_, to bear.

CYCADEae. A small and very anomalous order of flowering plants, chiefly found in Mexico, the East Indian Islands, South Africa, and Australia. They are Gymnogens as to ovules, and neither Exogens nor Endogens in the wood of their short, simple, or branched trunks, and they have dicotyledonous seeds. The leaves are pinnated (like those of cocoa-nut palms), and when young are rolled inwards as in Ferns.

The wood fibres are curiously perforated, and marked, by which they are recognized in a fossil state as well as by the trunk and foliage, and the cones, which contain the male flowers. The term is derived from ???a?, _cycas_, a name applied by the ancient Greek naturalist Threophrastus to a palm.

CYPERACEae. A tribe of plants answering to the English sedges; they are distinguished from gra.s.ses by their stems being solid, and generally triangular, instead of being hollow and round. Together with _Gramineae_, they const.i.tute what writers on botanical geography often call _glumaceae_.

DEBACLE. A great rush of waters, which, breaking down all opposing barriers, carries forward the broken fragments of rocks, and spreads them in its course. _Etym._, _debacler_, French, to unbar, to break up as a river does at the cessation of a long-continued frost.

DELTA. When a great river, before it enters the sea, divides into separate streams, they often diverge and form two sides of a triangle, the sea being the base. The land included by the three lines, and which is invariably alluvial, was first called, in the case of the Nile a delta, from its resemblance to the letter of the Greek alphabet which goes by that name ?. Geologists apply the term to alluvial land formed by a river at its mouth, without reference to its precise shape.

DENUDATION. The carrying away by the action of running water of a portion of the solid materials of the land, by which inferior rocks are laid bare. _Etym._, _denudo_, to lay bare.

DEOXIDIZED, DEOXIDATED. Deprived of oxygen. Disunited from oxygen.

DESICCATION. The art of drying up. _Etym._, _desicco_, to dry up.

DETRITUS. Matter worn or rubbed off from rocks. _Etym._, _de_, from, and _tero_, to rub.

DICOTYLEDONOUS. A grand division of the vegetable kingdom, founded on the plant having two _cotyledons_, or seed-lobes. _Etym._, d??, _dis_, double, and ??t???d??, _cotyledon_.

DIKES. When a ma.s.s of the unstratified or igneous rocks, such as granite, trap, and lava, appears as if injected into a rent in the stratified rocks, cutting across the strata, it forms a dike. They are sometimes seen running along the ground, and projecting, like a wall, from the softer strata on both sides of them having wasted away; whence they were first called in the north of England and in Scotland _dikes_, a provincial name for wall. It is not easy to draw the line between dikes and veins. The former are generally of larger dimensions, and have their sides parallel for considerable distances; while veins have generally many ramifications, and these often thin away into slender threads.

DILUVIUM. Those acc.u.mulations of gravel and loose materials, which, by some geologists, are said to have been produced by the action of a diluvian wave or deluge sweeping over the surface of the earth.

_Etym._, _diluvium_, deluge.

DIP. When a stratum does not lie horizontally, but is inclined, it is said to dip towards some point of the compa.s.s, and the angle it makes with the horizon is called the angle of dip or inclination.

DIPTERA. An order of insects, comprising those which have only two wings. _Etym._, d??, _dis_, double, and pte???, _pteron_, wing.

DOLERITE. One of the varieties of the Trap-rocks, composed of augite and felspar.

DOLOMITE. A crystalline limestone, containing magnesia as a const.i.tuent part. Named after the French geologist Dolomieu.

DUNES. Low hills of blown sand that skirt the sh.o.r.es of Holland, England, Spain, and other countries.