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[note.—The names of the subdivisions of the various systems given in this table are those generally accepted. Many, it will be seen, are of English origin ; others are foreign. Beside some of the latter the English equivalents (which are still current) are placed within parenthesis. A few German equivalents are given because reference is made to them in the text.]


Abrasion: the operation of wearing away by aqueous or glacial action.

Acid igneous rocks: rocks which contain a large percentage of silica to a small percentage of bases.

Agglomerate: volcanic fragmental rock, consisting of large angular, subangular, and roughly rounded blocks, confusedly huddled together.

Alluvium : a deposit resulting from the action of rivers or of tidal currents.

Amygdaloidal (Gr. amygdalon, an almond; eidos, an appearance): applied to igneous rocks containing vesicular cavities which have become filled, or partially filled, with subsequently introduced minerals. The cavities are frequently almond-shaped; the mineral kernels are termed amygdules.

Anticline (Gr. anti, against ; klino, I lean): a geological structure in which strata are inclined in opposite directions from a common axis; i. e., in a roof-like form. When its axis is vertical, an anticline is symmetrical; in an unsymmetricel anticline the axis is inclined.

Archsean : synonymous with Pre-Cambrian. See Table of Geological Systems.

Arenaceous: applied to strata which are largely or wholly composed of sand.

Argillaceous: applied to rocks composed of clay, or in which a notable proportion of clay is present.

Ash, volcanic: the finest-grained materials ejected during volcanic eruptions.

Basalt: a dark, hemicrystalline, basic igneous rock.

Base-level of Erosion: that level to which all lands tend to be reduced by

denudation. A land bast-levelled is usually very slightly above the sea-level,

and shows a gently undulating or approximately flat surface. Basic igneous rocks: rocks which contain a large percentage of bases to a

low percentage of silicic acid. Beaches, raised: former sea-margins ; sometimes appear as terraces of gravel,

sand, etc., sometimes as shelves cut in solid rock ; occur at all levels, from

a few feet up to several hundred yards above the sea.

Biotite {Biol. French physicist): a black or dark-green mica; occurs as a constituent of many crystalline igneous and schistose rocks.

Bombs, Tolcanic: clots of molten lava shot into the air from a volcano; having a rotatory motion, they acquire circular or elliptical forms, and are often vesicular internally, or hollow.

Bosses: large amorphous masses of crystalline igneous rock which have cooled and consolidated at some depth from the surface, and are now exposed by denudation.

Boulder-clay: typically, an unstratified clay more or less abundantly charged with angular and subangular stones of all shapes and sizes up to large blocks; the bottom or ground-moraine of prehistoric glaciers and ice-sheets.

Bunter (Ger. bunt, variegated): one of the subdivisions of the Triassic system; the sandstones of the Bunter are often spotted or mottled.

Bnttes (Fr.) and mesas (Sp.): names given, in the Territories of the United States, to conspicuous and more or less isolated hills and mountains. Suites are usually craggy, precipitous, and irregular in outline; mesas are flat-topped or tabular.

Cainozoic (Gr. tainos, recent; zee, life). See Table of Geological Systems. Calciferous : applied to strata which contain carbonate of lime as a binding or

cementing material ; or to strata among which numerous beds of limestone,

or other calcareous rocks, occur. Calc-sinter (Ger. talk (calx), lime ; sinter, a stalactite): a deposit from water

holding carbonate of lime in solution. Cambrian (Cambria or Wales): name given by Professor Sedgwick to one of

the Palaeozoic systems which was first carefully studied in Wales. Carboniferous: name given to the great coal-bearing system of the Palaeozoic

rocks. Chalybeate (L. chalybs, steel): applied to water impregnated with oxide of

iron. Chlorite (L. chloritis): a greenish mineral present in some schistose rocks ; often

occurs in igneous rocks as a product of alteration. Clastic (Gr. klastos, broken): applied to rocks composed of fragmental materials. Clinkers (Hut. tlinktr, thai which sounds): the cindery-like masses forming

the crust of some kinds of lava. Concretion: a body formed by irregular aggregation or accretion of mineral

matter, very often round a nucleus ; may be spherical, elliptical, or quite irregular and amorphous. Concretionary, formed of or containing concretions. Coulee (!■-.)j a stream of lava, whether flowing or become solid.

Crag-and-tail: a hill or crag showing an abrupt and often precipitous face on one side, and sloping away gradually to the low ground in the opposite direction.

Cretaceous: name given to the great chalk-bearing system of the Mesozoic strata.

Crust of the Earth: the outer portion of the earth which is accessible to geological investigation.

Curve of Erosion: A typical river has its steep mountain-track, its moderate valley-track, and its gentle plain-track. In the case of young rivers, the change from the one track to the other is often abrupt. In older rivercourses, such irregularities tend to be more and more reduced—the transition from the one track to the other becomes gradual—until eventually the course may be represented by a single curve, flattening out as it descends from source to mouth. This is the curve of erosion.

Debacle (F.): a tumultuous rush of water, sweeping forward rock cUbris, etc.

Deflation: the denuding and transporting action of the wind.

Degradation: the wasting or wearing down of the land by epigene agents.

Denudation: the laying bare of underlying rocks by the removal of superficial matter; the process by which the earth■s surface is broken up and the materials carried away.

Derivative rocks: rocks which have been formed out of the materials of preexisting minerals, rocks, and organic remains.

Detritus: any accumulation of materials formed by the breaking-up and wearing-away of minerals and rocks.

Devonian : name given to one of the Palaeozoic systems ; it is well developed in Devonshire.

Diluvium: name given to all coarse superficial accumulations which were formerly supposed to have resulted from a general deluge; now employed as a general term for all the glacial and fluvio-glacial deposits of the Ice Age.

Diorite (Gr. dioros, a boundary between): a crystalline igneous rock, belonging to a group intermediate in composition between the basic and acid groups.

Dogger: one of the subdivisions of the Jurassic system in Germany, etc.

Dolerite (Gr. doleros, deceptive): a crystalline basic igneous rock.

Dolina (It.): name given to the funnel-shaped cavities which communicate with the underground drainage-system in limestone regions. Similar cavities are known in this country as sinks and swallow-holes.

Dolomite (Dolomieu, the French geologist): carbonate of calcium and magnesium ; occurs as a crystallised mineral, and also as a granular crystalline rock (magnesian limestone).

Drum, Drumlin (Ir. and Gael, druman, the back, a ridge): a ridge or bank of boulder-clay alone, or of "rock " and boulder-clay. Ridges of this kind often occur numerously. There seem to be two varieties—(a) long parallel ridges or banks, and (b) short lenticular hillocks; the former usually consist of glacial accumulations alone ; the latter not infrequently contain a coreor nucleus of solid rock, or they may show solid rock at one end and glacial materials at the other.

Dyas (LL. the number two): name sometimes applied to the Permian system with reference to its subdivision into two principal groups.

Eocene (Gr. eos, dawn ; kainos, recent): see Table of Geological Systems.

Epigene (Gr. epi, upon; gennao, I produce): applied to the action of all the geological agents of change operating at or upon the earth■s surface; also to all accumulations formed by the action of those agents.

Erratics: boulders and fragments of rock which have been transported, generally by the agency of glaciers or floating ice, and are therefore foreign to the places in which they occur.

Eruptive rocks: massive igneous rocks generally; properly only those which have been extruded at the surface are truly eruptive; molten masses which have been intruded in the crust, and therefore below the surface, are irruptive.

Eskers (Ir. eiscir, a ridge): ridges of gravel and sand which appear to have been formed in tunnels underneath the great glaciers and ice-sheets of former times; same as the Swedish osar.

Felspars (Ger. /eld, a field ; spath, spar): a group of minerals, common constituents of many igneous and schistose rocks.

Fire-clay: properly a clay suitable for the manufacture of fire-bricks; in geology is applied to the argillaceous layer underlying most coal-seams, which consists generally of some kind of clay, but is not always suitable for firebricks.

Fluvio-glacial: applied to sedimentary deposits resulting from the action of streams and rivers escaping from a glacier or an ice-sheet.

Foliated rocks: another name for schist and schistose rocks. See Schist.

Formation: a series of rocks having some character in common, whether of origin, age, or composition; often applied to a group of strata containing a

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