[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.] GLOSSARY 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. Archæan: 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 base-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 (Biot, French physicist): a black or dark-green mica; occurs as a constituent of many crystalline igneous and schistose rocks. Bombs, volcanic: 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. Buttes (Fr.) and mesas (Sp.): names given, in the Territories of the United States, to conspicuous and more or less isolated hills and mountains. Buttes are usually craggy, precipitous, and irregular in outline; mesas are flat-topped or tabular. Cainozoic (Gr. kainos, recent; zoe, 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, kalk (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 Paleozoic systems which was first carefully studied in Wales. Carboniferous: name given to the great coal-bearing system of the Paleozoic 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 (Dut. klinker, that 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. Coulée (F.): 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. Débâcle (F.): a tumultuous rush of water, sweeping forward rock débris, 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 Palæozoic systems; it is well developed in Devonshire. Diluvium: name given to all coarse superficial accumulations which were for merly 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. |