|
BASALT , in See also:petrology, one of the See also:oldest See also:rock names, supposed to be derived from an Ethiopian word basal, signifying a See also:- STONE
- STONE (0. Eng. shin; the word is common to Teutonic languages, cf. Ger. Stein, Du. steen, Dan. and Swed. sten; the root is also seen in Gr. aria, pebble)
- STONE, CHARLES POMEROY (1824-1887)
- STONE, EDWARD JAMES (1831-1897)
- STONE, FRANK (1800-1859)
- STONE, GEORGE (1708—1764)
- STONE, LUCY [BLACKWELL] (1818-1893)
- STONE, MARCUS (184o— )
- STONE, NICHOLAS (1586-1647)
stone which yields See also:iron; according to See also:Pliny, the first basalts were obtained in See also:Ethiopia. In current usage the See also:term includes a large variety of types of igneous rock belonging to the basic subdivision, dark in See also:colour weathering to See also:- BROWN
- BROWN, CHARLES BROCKDEN (1771-181o)
- BROWN, FORD MADOX (1821-1893)
- BROWN, FRANCIS (1849- )
- BROWN, GEORGE (1818-188o)
- BROWN, HENRY KIRKE (1814-1886)
- BROWN, JACOB (1775–1828)
- BROWN, JOHN (1715–1766)
- BROWN, JOHN (1722-1787)
- BROWN, JOHN (1735–1788)
- BROWN, JOHN (1784–1858)
- BROWN, JOHN (1800-1859)
- BROWN, JOHN (1810—1882)
- BROWN, JOHN GEORGE (1831— )
- BROWN, ROBERT (1773-1858)
- BROWN, SAMUEL MORISON (1817—1856)
- BROWN, SIR GEORGE (1790-1865)
- BROWN, SIR JOHN (1816-1896)
- BROWN, SIR WILLIAM, BART
- BROWN, THOMAS (1663-1704)
- BROWN, THOMAS (1778-1820)
- BROWN, THOMAS EDWARD (1830-1897)
- BROWN, WILLIAM LAURENCE (1755–1830)
brown, and comparatively See also:rich in See also:magnesia and iron. Somebasalts are in large measure glassy (tachylites), and many are very See also:fine grained and compact; but it is more usual for them to exhibit porphyritic structure, showing larger crystals of See also:olivine, See also:augite or See also:felspar in a finely crystalline groundmass. Olivine and augite are the commonest porphyritic minerals in basalts, the former See also:green or yellowish (and weathering to green or brown See also:serpentine), the latter See also:pitch-See also:black. Porphyritic See also:plagioclase felspars, however, are also very See also:common, and may be one or two inches in length, though usually not exceeding a See also:quarter of an See also:inch; when fresh they are dark See also:grey with smooth lustrous cleavage surfaces; when decomposed they become turbid, and assume grey or greenish shades. Basaltic lavas are frequently spongy or pumiceous, especially near their surfaces; and, in course of See also:- TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
- TIME, MEASUREMENT OF
- TIME, STANDARD
time, the See also:steam cavities become filled with secondary minerals such as See also:calcite, See also:chlorite and See also:zeolites. Another characteristic of this See also:group of rocks is the perfection with which many of them show prismatic or columnar jointing, a structure often called " basaltic jointing."
The minerals of basaltic rocks have a fairly See also:uniform See also:character throughout the whole group. In microscopic See also:section the olivine is See also:pale green or colourless, and is very frequently more or less altered to serpentine. The secondary See also:mineral begins to See also:form upon the surfaces and along the cracks of the olivine, gradually producing a mesh-See also:work in the interstices of which small kernels of olivine remain; and when the See also:process is completed the mesh structure persists in the resulting pseudomorph, giving a clear indication as to its See also:history. The augite is mostly brown, often with a purplish tinge, hardly at all dichroic, but frequently showing zonal or See also:hour-See also:glass structure, and various types of twinning. It weathers to chlorite, uralite, calcite, &c. The plagioclase felspar, if fresh, is transparent and appears See also:simple in See also:ordinary See also:light, but when polarized breaks up into a See also:series of bars of different See also:colours owing to its complex twinned structure.
Practically all varieties of this mineral from See also:anorthite to See also:albite are known to occur in basalt, but by far the commonest See also:species are See also:bytownite and See also:labradorite. Weathering destroys the limpid character of the fresh mineral, producing turbid pseudomorphs containing See also:epidote, calcite, See also:- WHITE
- WHITE, ANDREW DICKSON (1832– )
- WHITE, GILBERT (1720–1793)
- WHITE, HENRY KIRKE (1785-1806)
- WHITE, HUGH LAWSON (1773-1840)
- WHITE, JOSEPH BLANCO (1775-1841)
- WHITE, RICHARD GRANT (1822-1885)
- WHITE, ROBERT (1645-1704)
- WHITE, SIR GEORGE STUART (1835– )
- WHITE, SIR THOMAS (1492-1567)
- WHITE, SIR WILLIAM ARTHUR (1824--1891)
- WHITE, SIR WILLIAM HENRY (1845– )
- WHITE, THOMAS (1628-1698)
- WHITE, THOMAS (c. 1550-1624)
white micas, See also:kaolin, &c. When these minerals occur as phenocrysts their crystalline outlines may be very perfect (though, especially in the olivine, corrosion and partial resorption may give rise to rounded or irregular forms).
In the groundmass, or second See also:generation of crystal, not only are the ingredients smaller, but their crystals are less perfect; yet in many basalts small See also:lath-shaped felspars and See also:minute prisms ofaugite, densely crowded together, form the See also:matrix. With these there may be a greater or less amount of brown, isotropic glass. Olivine rarely occurs as an ingredient of the groundmass. In the vitreous basalts sometimes very few crystallized minerals are observable; the greater See also:part of the rock is a dark brown glassy material, almost opaque even in the thinnest sections, and generally charged with black grains of See also:magnetite, See also:skeleton crystals of augite or felspar, See also:spherulites, perlitic cracks, or steam vesicles. In other basaltic rocks no glassy material appears, but the whole See also:mass is thoroughly crystallized; rocks of this nature are generally known to See also:British petrologists as dolerites (q.v.). Till See also:recent years it was widely believed by See also:continental geologists that the pre-See also:Tertiary basalts differed so fundamentally from their Tertiary and recent representatives that they were entitled to be regarded as a distinct class. For the older rocks the names anamesite, See also:diabase porphyrite, diabas-mandel-See also:stein, or melaphyre were used, and are still favoured by many writers, to indicate varieties and states of more or less altered basalts and dolerites, though no longer held to differ in any essential respects from the better preserved basalts. Still older is the term See also:trap, which is derived from a See also:Swedish word meaning " a See also:stair," for in many places superposed sheets of basalt See also:weather with well-marked step-like or terraced features. This designation is still used as a See also:general term for the whole See also:suite of basaltic rocks by many geologists and travellers (e.g. trap-dikes, the " traps " of the See also:Deccan).
In the See also:early years of the loth See also:century a See also:great controversy convulsed the See also:geological See also:world as to the origin of the older basalts or " floetz-traps." See also:Werner, the Saxon mineralogist, and his school held them to be of aqueous origin, the chemical precipitates deposited in primeval seas, but See also:Hutton and a number of See also:French geologists maintained that they were really volcanic rocks emitted by craters now See also:extinct (see See also:GEOLOGY: See also:Historical).
Of the less common minerals of basalt, a few may be mentioned. Black See also:hornblende, dark brown in thin sections, and often corroded, is not uncommon, especially in intrusive basalts. See also:Hypersthene occurs also, usually replacing olivine. Black See also:mica (See also:biotite) is not infrequently to be seen. See also:Sapphire, See also:garnet and See also:zircon are rare. Minerals of the felspathoid group occur in a large number of basaltic rocks; See also:nepheline and See also:leucite are the most common, but haiiyne is occasionally See also:present. If nepheline entirely replaces felspar, the rock is known as nepheline-basalt; if the replacement is only partial the term nepheline-basanite is used. Similarly there are leucite-basalts and leucite-basanites. The nepheline is in small six-sized prisms, and usually cannot be detected with the unaided See also:eye. Even with the help of the See also:microscope nepheline basalts are not always easy to determine, as the crystals may be exceedingly small and imperfect, and they readily decompose into See also:analcite and zeolites. In some cases only the presence of an anisotropic substance, with weak See also:double See also:refraction and readily attacked by acids (the so-called " nephelinitoid "), can be made out.
This substance may be imperfectly crystallized nepheline, or a See also:peculiar glass which is rich in soda. Most nepheline basalts are fine grained, very dark coloured rocks, and belong to the Tertiary See also:period. They are fairly common in some parts of See also:Germany and occur also in See also:Tripoli, See also:Asia See also:Minor, See also:Montana, Cape Verde Islands, &c. Leucite-basalts contain small rounded crystals of leucite in See also:place of plagioclase felspar. Rocks of this group are well known in the See also:Eifel, and other volcanic districts in Germany, also in Bohemia, See also:Italy, See also:Java, Montana, See also:Celebes, &c. The minerals hauyne, nosean, See also:sodalite and melilite tend to occur with some frequency in nepheline and leucite-basalts, though rare in ordinary basalts. Melilite, a See also:lime-alumina-silicate, is characteristic of certain very basic rocks, the melilite-basalts. It is pale yellow or colourless in thin sections, and yields peculiar and characteristic dark See also:blue polarization colours. This rare group of rocks is known to occur in Bohemia, See also:Swabia and See also:South See also:Africa. Perofskite, in small dark brown cubic crystals, is a See also:constant See also:accessory in these rocks. The augite is usually See also:violet coloured, and shows zonal and hour-glass structures. Green augite may occur in the nepheline-basalts, and aegerine (soda-iron-augite) is occasionally found in them.
The See also:distribution of basalts is world-wide; and in some places
they occur in immense masses, and See also:cover great areas. In See also:Washington, See also:Oregon, and See also:Idaho many thousands of square See also:miles are occupied by basaltic-See also:lava flows. In the See also:Sandwich Islands and See also:Iceland they are the prevalent lavas; and the well-known columnar jointed basalts of See also:Skye, See also:Staffa, and See also:Antrim (See also:Giant's See also:Causeway) form a southward See also:extension of the Icelandic volcanic See also:province, with which they are connected by the similar rocks of the See also:Faeroe Islands. In the Deccan in See also:India great basaltic lava See also:fields are known; and See also:Etna and See also:Vesuvius emit basaltic rocks. In older geological periods they were not less common; for example, in the Carboniferous in See also:Scotland. (J. S.
End of Article: BASALT
Additional information and Comments
There are no comments yet for this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.
Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.
Links to articles and home page are always encouraged.
|
del.icio.us it!