Principles of Geology Part 97

EARTH'S CRUST. Such superficial parts of our planet as are accessible to human observation.

EOPYROSIS. A Greek term for a destruction by fire.

ELYTRA. The wing-sheaths, or upper crustaceous membranes, which form the superior wings in the tribe of beetles. They cover the body, and protect the true membranous wing. _Etym._, e??t???, _elytron_, a sheath.

ENDOGENS. A cla.s.s of flowering plants, whose stems present no distinction of wood, pith, and bark. The wood is disposed in bundles, placed nearer the axis than those of the previous year, as in palm trunks. This cla.s.s answers to the Monocotyledones of Jussieu. _Etym._, e?d??, _endon_, within, and ?e?es??, _genesis_, increase.

ENTOMOSTRACA. Cuvier's second section of Crustacea; so called from their relationship to insects. _Etym._, e?t?a, _entoma_, insects.

EOCENE. A name given to the lowest division of the tertiary strata, containing an extremely small percentage of living species amongst its fossil sh.e.l.ls, which indicate the first commencement or dawn of the existing state of the animate creation. _Etym._, ???, _eos_, aurora or the dawn, and ?a????, _kainos_, recent.

ESCARPMENT. The abrupt face of a ridge of high land. _Etym._, _escarper_, French, to cut steep.

ESTUARIES. Inlets of the land, which are entered both by rivers and the tides of the sea. Thus we have the estuaries of the Thames, Severn, Tay, &c. _Etym._, _aestus_, the tide.

EXOGENS. A cla.s.s of flowering plants whose stems have bark, wood, and pith. The bark is increased by layers deposited within the previously formed layers and the wood of layers or rings placed outside of those of the previous year. This cla.s.s answers to the Dicotyledones of Jussieu, and includes all common English trees except pines, &c. (See GYMNOGENS.) _Etym._, e??, _exo_, outside, ?e?es??, _genesis_, increase.

EXPERIMENTUM CRUCIS. A decisive experiment, so called, because, like a cross or direction-post, it directs men to true knowledge; or, as some explain it, because it is a kind of torture whereby the nature of the thing is extorted, as it were, by violence.

EXUVIae. Properly speaking, the transient parts of certain animals which they put off or lay down to a.s.sume new ones, as serpents and caterpillars shift their skins; but in geology it refers not only to the cast-off coverings of animals, but to fossil sh.e.l.ls and other remains which animals have left in the strata of the earth. _Etym._, _exuere_, to put off or divest.

FALUNS. A French provincial name for some tertiary strata abounding in sh.e.l.ls in Touraine, which resemble in lithological characters the "Crag" of Norfolk and Suffolk.

FAULT, in the language of miners, is the sudden interruption of the continuity of strata in the same plane, accompanied by a crack or fissure, varying in width from a mere line to several feet, which is generally filled with broken stone, clay, &c.

[Ill.u.s.tration: Fig. 99.]

The strata, _a_, _b_, _c_, &c., must at one time have been continuous; but a fracture having taken place at the fault F, either by the upheaving of the portion A, or the sinking of the portion B, the strata were so displaced that the bed _a_ in B is many feet lower than the same bed _a_ in the portion A.

FAUNA. The various kinds of animals peculiar to a country const.i.tute its FAUNA, as the various kinds of plants const.i.tute its FLORA. The term is derived from the FAUNI, or rural deities, in Roman Mythology.

FELSPAR. A simple mineral, which, next to quartz, const.i.tutes the chief material of rocks. The white angular portions in granite are felspar. This mineral always contains some alkali in its composition. In _common felspar_ the alkali is potash; in another variety, called Albite or Cleavlandite, it is soda. Gla.s.sy felspar is a term applied when the crystals have a considerable degree of transparency. _Compact_ felspar is a name of more vague signification. The substance so called appears to contain both potash and soda.

FELSPATHIC. Of or belonging to felspar.

FERRUGINOUS. Any thing containing iron. _Etym._, _ferrum_, iron.

FISSILE, easily cleft, dividing readily into an indefinite number of parallel laminae, like slates.

FLOETZ ROCKS. A German term applied to the secondary strata by the geologists of that country, because these rocks were supposed to occur most frequently in flat horizontal beds. _Etym._, _flotz_, a layer or stratum.

FLORA. The various kinds of trees and plants found in any country const.i.tute the FLORA of that country in the language of botanists.

FLUVIATILE. Belonging to a river. _Etym._, _fluvius_, a river.

FORAMINIFERA. A name given by D'Orbigny to a family of microscopic sh.e.l.ls. Their different chambers are united by a small perforation or _foramen_. Recent observation has shown that some at least are not Cephalopoda, as D'Orbigny supposed.

FORMATION. A group, whether of alluvial deposits, sedimentary strata, or igneous rocks, referred to a common origin or period.

FOSSIL. All minerals were once called fossils, but geologists now use the word only to express the remains of animals and plants found buried in the earth. _Etym._, _fossilis_, any thing that may be dug out of the earth.

FOSSILIFEROUS. Containing organic remains.

GALENA. A metallic ore, a compound of lead and sulphur. It has often the appearance of highly polished lead. _Etym._, ?a?e?, _galeo_, to shine.

GARNET. A simple mineral, generally of a deep red color, crystallized; most commonly met with in mica slate, but also in granite and other igneous rocks.

GASTEROPODS. A division of the Testacea, in which, as in the limpet, the foot is attached to the body. _Etym._, ?ast??, _gaster_, belly, and p?da, _poda_, feet.

GAULT. A provincial name in the east of England for a series of beds of clay and marl, the geological position of which is between the Upper and Lower Greensand.

GAVIAL. A kind of crocodile found in India.

GEM, or GEMMULE, from the Latin _gemma_, a bud. The term, applied to zoophytes, means a young animal not confined within an envelope or egg.

GEOLOGY, GEOGNOSY. Both mean the same thing; but with an unnecessary degree of refinement in terms, it has been proposed to call our description of the structure of the earth _geognosy_ (_Etym._, ?ea, _gea_, earth, and ????sc?, _ginosco_, to know), and our theoretical speculations as to its formation _geology_ (_Etym._, ?ea, and ?????, _logos_, a discourse).

GLACIER. Vast acc.u.mulations of ice and hardened snow in the Alps and other lofty mountains. _Etym._, _glace_, French for ice.

GLACIS. A term borrowed from the language of fortification, where it means an easy insensible slope or declivity, less steep than a _talus_, which see.

GNEISS. A stratified primary rock, composed of the same materials as granite, but having usually a larger proportion of mica and a laminated texture. The word is a German miner's term.

GRAMINEae. The order of plants to which gra.s.ses belong. _Etym._, _gramen_, gra.s.s.

GRANITE. An unstratified or igneous rock, generally found inferior to or a.s.sociated with the oldest of the stratified rocks, and sometimes penetrating them in the form of dikes and veins. It is usually composed of three simple minerals, felspar, quartz, and mica, and derives its name from having a coa.r.s.e _granular_ structure; _granum_, Latin for grain. Waterloo bridge, and the paving-stones in the carriage-way of the London streets, afford good examples of the most common varieties of granite.

GREENSAND. Beds of sand, sandstone, limestone, belonging to the Cretaceous Period. The name is given to these beds because they often, but not always, contain an abundance of green earth or chlorite scattered through the substance of the sandstone, limestone, &c.

GREENSTONE. A variety of trap, composed of hornblende and felspar.

GREYWACKe. _Grauwacke_, a German name, generally adopted by geologists for some of the most ancient fossiliferous strata. The rock is very often of a gray color; hence the name, _grau_, being German for gray, and _wacke_, being a provincial miner's term.

GRIT. A provincial name for a coa.r.s.e-grained sandstone.

GYMNOSPERMOUS. _Etym._, ?????, _gymnos_, naked, and spe?a, _sperma_, a seed. (See GYMNOGENS.)

GYMNOGENS. A cla.s.s of flowering plants, in which the ovules are not inclosed in an ovary. They are also called _gymnosperms_, the seeds in like manner not being inclosed in a pericarp. It includes all _Coniferae_, as pine, fir, juniper, cypress, yew, cedar, &c., and _Cycadeae_. All are Dicotyledonous (a few have many cotyledons), and all Exogenous, except _Cycas_, the growth of which is anomalous. The term is applied in contradistinction to _Angiosperms_, which see.

_Etym._, ?????, naked, and ?e?es??, increase.

GYPSUM. A mineral composed of lime and sulphuric acid, hence called also _sulphate_ of _lime_. Plaster and stucco are obtained by exposing gypsum to a strong heat. It is found so abundantly near Paris, that plaster of Paris is a common term in this country for the white powder of which casts are made. The term is used by Pliny for a stone used for the same purposes by the ancients. The derivation is unknown.

GYPSEOUS, of or belonging to gypsum.

GYROGONITES. Bodies found in freshwater deposits, originally supposed to be microscopic sh.e.l.ls, but subsequently discovered to be seed-vessels of freshwater plants of the genus _Chara_. See above p. 742. _Etym._, ?????, _gyros_, curved, and ?????, _gonos_, seed, on account of their external structure.