Page images
PDF
EPUB

well-marked and distinctive assemblage of fossils—a group of subordinate importance to a system. Fragmental rocks: see Clastic and Derivative.

Gabbro (It.): a coarsely crystalline basic igneous rock.

Geanticline (Gr. ge, the earth; and F. anticline): a broad or regional arching or bending up of the crust—thus, a geanticline may be composed of strata showing all kinds of geological structure. It is simply a bulging or swelling up of the crust which affects a wide region. Geosyncline is just the opposite: it is a wide or broad region of depression, i. e., a sinking of the earth's crust as a whole.

Geysers (Icel.): eruptive fountains of hot water and steam.

Giants' kettles: large pot-holes often observed in the deserted beds of old glaciers ; they are believed to have been drilled by water descending from the surface of the glaciers and setting stones and boulders in rapid rotation.

Glacial Period: the deposits of the Ice Age referred to in the text belong for the most part to the Pleistocene system. Cold climatic conditions, however, had set in before the close of the Pliocene, and were continued into the Recent period—the last of our snow-fields and glaciers having vanished during the formation of some of the youngest raised beaches—a time when Neolithic man lived in Britain.

Gneiss (Ger.); one of the more coarsely crystalline schistose rocks.

Granite (It. granite): one of the deep-seated plutonic crystalline igneous rocks. Granitoid, having the structure of granite.

Greywacke (Ger. grauwacki): a sedimentary rock, somewhat metamorphosed; common in the Palaeozoic systems.

Grit: generally a coarse-grained arenaceous rock; the harder kinds are used for grindstones.

Ground-moraine: the rock-rubbish formed by the grinding action of glaciers and ice-sheets.

Gypsum (Gr. gypsos, chalk): a crystalline mineral composed of sulphate of lime.

Hade: a miner's term for the inclination or deviation of a lode or fault from

the vertical. Haematite (Gr. haimatites, blood-like): a mineral compound of oxide of

iron, which yields a blood-red streak when scratched. Holocrystalline (Gr. holes, whole; F. crystalline). applied to igneous rocks

composed entirely of crystalline ingredients, as granite.

Hornblende (Ger. horn, horn ; blendtn, to dazzle): a mineral constituent of many crystalline igneous and schistose rocks.

Horste: name given by German geologists to isolated mountains severed by dislocations from rock-masses with which they were formerly continuous, but which have since subsided to a lower level. A■ump/gebirge (lit., rumpmountains) is another name for this type of mountain. .

Humous acids: general name for the various acids met with in the humus or vegetable mould, and which are derived from the decomposition of organic matter.

Hypogene (Gr. hypo, under ; gennao, I produce): applied to geological action under the earth■s surface, and to the products of that action; opposed to Epigene (a. v.).

Infraglacial: applied to deposits formed and accumulated underneath, or in

the bottom parts of, glaciers and ice-sheets; and to the geological action

of the ice upon rocks over which it flows. In situ: in its original situation; applied to minerals, fossils, and rocks which

occupy their natural place or position. Insolation: the geological action of the sun■s heat upon rocks at the surface. Intraglacial: applied to rock-fragments embedded in the central and upper

portions of glaciers and ice-sheets. Intrusive rocks: molten rocks which have been injected among pre-existing

rock-masses. Inversion: a geological structure in which strata have been so folded as to be

turned upside down. Isoclinal (Gr. isos, equal ; klino, to lean): applied to strata folded in a series

of unsymmetrical anticlines and synclines whose axes all incline in one and

the same direction.

Joints: natural division-planes which intersect bedded and amorphous rocks of all kinds. In bedded rocks two sets of joints are usually recognisable (master-joints), which cut each other at approximately right angles. In crystalline igneous and schistose rocks the joints as a rule are somewhat irregular; but to this there are exceptions—as in certain granites, basalts, etc.—many of the fine-grained igneous rocks showing prismatic jointing or columnar structure.

Jurassic (from ■Jura Mountains): one of the Mesozoic systems.

Karnes: ridges and mounds of gravel and sand generally, but now and again of rude rock-rubbish. They are of glacial and fluvio-glacial origin, having been accumulated, in many cases, along the terminal margins of large glaciers and ice-sheets.

Kaolin (Chin, kaoling); a fine clay resulting from the chemical decomposition

of felspar. Keuper (Ger.): one of the subdivisions of the Triassic system.

Laccolith (Gr. lakkos, a cistern; lithos, stone): name given to intrusive rocks which, when rising from below, have spread out laterally, so as to form lenticular masses, thereby lifting the rocks above them so as to form domeshaped swellings at the surface.

Lapilli(L.): small stones ejected from volcanoes in eruption.

Lee-seite (Ger.): the side of a hill or prominent rock in a glaciated region which has been sheltered or protected by its position from the abrading action of the ice-flow. The opposite side, exposed to that action, and therefore " glaciated," is termed the Stoss-seite.

Lias: one of the subdivisions of the Jurassic system.

Lignite: brown coal, not so highly mineralised as common coal.

Maars: name given in the Eifel to crater-lakes.

Macalubas: mud-volcanoes, so called from the well-known Macalube, near

Girgenti, in Sicily.
Magma: the molten or plastic material which, when cooled and solidified,

forms crystalline, hemicrystalline, or glassy igneous rocks.
Malm: one of the subdivisions of the Jurassic system in Germany, etc.
Master-joint: see Joints.
Mesa: see Buttes.

Mesozoic (Gr. mesos, middle; zoe, life): see Table of Geological Systems.
Metaraorphic (Gr. meta, expressing change ; morphi, form): applied to rocks

which have been more or less completely changed in form and structure—

their constituent materials having been rearranged.
Mica: a group of minerals, common constituents of many igneous and schistose

rocks.
Millstone Grit: one of the subdivisions of the Carboniferous system.
Miocene (Gr. meion, less; Aainos, recent): one of the Cainozoic systems.
Monocline (Gr. monos, single ; klino, to lean): the simplest kind of fold ; an

abrupt increase of dip in gently inclined or approximately horizontal strata,

followed by an equally abrupt return to the original position. Moulin (F., a mill): an approximately vertical cavity or shaft worked out in

a glacier by water descending from the surface through a crevasse. See

Giants■ kettles.

[ocr errors]

Necks: plugged-up pipes of volcanic eruption ; the throats of old volcanoes

v. h:ch have been laid bare by denudation. fieri (K. ): granular snow; the condition assumed by snow on its passage into

glacier-ice.

Obsidian: a volcanic glassy rock.

Old Red Sandstone: see Table of Geological Systems.

Oligocene (Gr. •.Y$v», little ; haimos, recent): one of the Cainozoic systems.

Olivine: a greenish mineral ; a common constituent of many basic igneous

rocks. Oolite (Gr. tvn, an egg; tithes, stone): a granular limestone, comn.on in tl

Jurassic system, which on this accoui.t used to be known as the Oolitic

formation. Osar (Swedish): see Eskers. Outlier: a detached mass of rock resting upon and surrounded on all sides by

older rocks. Overfold: an overturned or inverted fold ; the axis so inclined that one limb of

the fold is doubled back under the other. When the axis becomes horizontal, or nearly so, the fold is recumbent. Overthrust: a faulted overfold; the fold has been dislocated, and one limb

pushed over the other along a thrust-plane.

Paleozoic (Gr. paiaios, ancient ; toe, life): see Table of Geological Systems.

Parallel roads: old lake-beaches, seen in Glen Roy (Scottish Highlands) and other valleys in its neighbourhood.

Paysage morainique: a region abundantly covered with terminal moraines.

Perched blocks: boulders transported by glacier-ice and stranded in prominent positions.

Petrography (Gr. petros, a rock ; grapho, to describe): the study of rocks— Petrology and Lithology.

Phonolite (Clinkstone) (Gr. phone, sound; lilhos, stone): a volcanic crystalline rock ; when fresh and compact it has a metallic ring or clink under the hammer.

Pleistocene (Gr. pleistos, most; hainos, recent): one of the Post-Tertiary systems.

Pliocene (Gr. p/eion, more; hainos, recent): one of the Cainozoic systems.

Plutonic (Pluto, the god of the infernal regions): applied to deep-seated igneous action ; also to deep-seated igneous rocks — those which have cooled and consolidated at some depth from the surface.

Post-Tertiary or Quaternary: the youngest group of systems. See Table.
Pre-Cambrian or Archaean: the oldest system of rocks.
Pumice: any froth-like, foam-like, spongy, porous, or cellular lava.
Pyroxene (Gr. pur, fire; xencs, a guest): a family of minerals, common con-
stituents of many crystalline igneous, and of some schistose rocks.

Quadersandstein: name given in Saxony, Bohemia, and Silesia to the Cretaceous system ; so called because the sandstone of which it is chiefly composed is traversed by abundant well-marked vertical joints, that cause the rock to weather into square, tabular, and pyramidal hills, and pillarlike masses.

Quaquaversal (L. quaqua, wheresoever; versus, turned): applied to strata which dip outwards in all directions from a common centre; dome-shaped strata.

Quartz (Ger.): common form of native silica ; the most common of all rockforming minerals.

Quaternary: alternative name for Post-Tertiary.

Raised beaches: see Beaches.

Recent period: the latest of the geological systems; passes gradually into the present or existing condition of the earth.

Reversed faults: in these the hade or inclination of the fault is in the direction of upthrow—lower rocks having been pushed over higher rocks. See Overthrust and Thrust-plane.

Revived rivers: when the rivers of a region have succeeded in cutting their channels down to the base-level, they have a slight fall and How sluggishly. Should the whole region then lie elevated, while the direction of its slopes remains unchanged, the erosive energy of the rivers is renewed, and they are said, therefore, to be revived.

Rhaeticffrom the Khaetian Alps): one of the subdivisions of theTriassic system.

Rhyolite (Gr. rheo, to flow ; lilhos, stone): an acid volcanic rock.

Roches moutonnees: rocks rounded like the back of a sheep ; name given to rocks which have been abraded, rounded, and smoothed by glacial action.

Rothliegendes (Ger.): one of the subdivisions of the Permian system.

Rumpfgebirge (Ger.): same as Horste (</. v.).

Salses (Fr.): another name for mud-volcanoes or Macalubas (q. v.).

Schist (Gr. schistos, easily split): a crystalline rock in which the constituent

minerals are arranged in rudely alternate parallel layers or folia; a foliated

rock.

« PreviousContinue »