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GNEISS

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Originally appearing in Volume V12, Page 150 of the 1911 Encyclopedia Britannica.
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GNEISS , a See also:

term See also:long used by the miners of the Harz Mountains to designate the See also:country See also:rock in which the See also:mineral See also:veins occur; it is believed to be a word of See also:Slavonic origin meaning " rotted " or " decomposed." It has gradually passed into See also:acceptance as a generic term signifying a large and varied See also:series of metamorphic rocks, which mostly consist of See also:quartz and See also:felspar (See also:orthoclase and See also:plagioclase) with See also:muscovite and See also:biotite, See also:hornblende or See also:augite, See also:iron oxides, See also:zircon and See also:apatite. There is also a long See also:list of See also:accessory minerals which are See also:present in gneisses with more or less frequency, but not invariably, as See also:garnet, See also:sillimanite, cordierite, See also:graphite and graphitoid, See also:epidote, See also:calcite, orthite, See also:tourmaline and See also:andalusite. The gneisses all possess a more or less marked parallel structure or foliation, which is the See also:main feature by which many of them are separated from the granites, a See also:group of rocks having nearly the same mineralogical See also:composition and closely allied to many gneisses. The felspars of the gneisses are predominantly orthoclase (often perthitic), but See also:microcline is See also:common in the more See also:acid types and See also:oligoclase occurs also very frequently, especially in certain sedimentary gneisses, while more basic varieties of plagioclase are rare. Quartz is very seldom absent and may be See also:blue or milky and opalescent. Muscovite and biotite may both occur in the same rock; in other cases only one of them is present. The commonest and most important types of gneiss are the micagneisses. Hornblende is See also:green, rarely brownish; augite See also:pale green or nearly colourless; See also:enstatite appears in some granulitegneisses. Epidote, often with enclosures of orthite, is by no means rare in gneisses from many different parts of the See also:world. Sillimanite and andalusite are not infrequent ingredients of gneiss, and their presence has been accounted for in more than one way. Cordierite-gneisses are a See also:special group of See also:great See also:interest and possessing many peculiarities; they are partly, if not entirely, foliated contact-altered sedimentary rocks. Kyanite and See also:staurolite may also be mentioned as occasionally occurring.

Many varieties of gneiss have received specific names according to the minerals they consist of and the structural peculiarities they exhibit. Muscovite-gneiss, biotite-gneiss and muscovitebiotite-gneiss, more common perhaps than all the others taken together, are See also:

grey or pinkish rocks according to the See also:colour of their prevalent felspar, not unlike granites, but on the whole more often See also:fine-grained (though coarse-grained types occur) and possessing a gneissose or foliated structure. The latter consists in the arrangement of the flakes of See also:mica in such a way that their faces are parallel, and hence the rock has the See also:property of splitting more readily in the direction in which the mica plates are disposed. This fissility, though usually marked, is not so great as in the See also:schists or slates, and the split faces are not so smooth as in these latter rocks. The films of mica may be continuous and are usually not See also:flat, but irregularly curved. In some gneisses the parallel flakes of mica are scattered through the quartz and felspar; in others these minerals See also:form discrete bands, the quartz and felspar being grouped into lenticles separated by thin films of mica. When large felspars, of rounded or elliptical form, are visible in the gneiss, it is said to have augen structure (Ger. Augen=eyes). It should also be remarked that the essential component minerals of the rocks of this See also:family are practically always determinable by naked See also:eye inspection or with the aid of a See also:simple See also:lens. If the rock is too fine grained for this it is generally relegated to the schists. When the bands of folia are very fine and tortuous the structure is called helizitic. In mica-gneisses sillimanite, kyanite, andalusite and garnet may occur.

The significance of these minerals is variously interpreted; they may indicate that the gneiss consists wholly or in See also:

part of sedimentary material which has been contact-altered, but they have also been regarded as having been See also:developed by metamorphic See also:action out of biotite or other See also:primary ingredients of the rock. Hornblende-gneisses are usually darker in colour and less fissile than mica-gneisses; they contain more plagioclase, less orthoclase and microcline, and more See also:sphene and epidote. Many of them are See also:rich in hornblende and thus form transitions to amphibolites. See also:Pyroxene-gneisses are less frequent but occur in many parts of both hemispheres. The " See also:charnockite " series are very closely allied to the pyroxene-gneisses. See also:Hypersthene and See also:scapolite both may occur in these rocks and they are some-times garnetiferous.and its elements are 'so completely confused that the geologist can no longer disentangle them. When we remember that 'in the earlier stages of the See also:earth's See also:history, to which most gneisses belong, and in the relatively deep parts of the earth's crust, where they usually occur, there has been most igneous injection and greatest frequency of earth movements, it is not difficult to understand the See also:geological See also:distribution of gneissose rocks. All the factors which are required for their See also:production, See also:heat, See also:movement, plutonic intrusions, contact alteration, interstitial moisture at high temperatures, are found at great depths and have acted most frequently and with greatest See also:power on the older rock masses. But locally, where the conditions were favourable, the same processes may have gone on in comparatively See also:recent times. Hence, though most gneisses are Archean, all gneisses are not necessarily so. (J. S.

End of Article: GNEISS

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