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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 (common in different forms to most European languages)

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 (through Fr. pal, from Lat. palms, a stake, for paglus, from the stem pag- of pangere, to fix; " pole " is from the same original source)

pale green or nearly colourless; See also:

ENSTATITE

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

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

special group of See also:

GREAT

great See also:

INTEREST

interest and possessing many peculiarities; they are partly, if not entirely, foliated contact-altered sedimentary rocks. Kyanite and See also:

STAUROLITE

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 (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour of their prevalent felspar, not unlike granites, but on the whole more often See also:

FINE

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

mica in such a way that their faces are parallel, and hence the rock has the See also:

PROPERTY

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 (Gr. vxq'ecv, to split)

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 (a modification of O. Eng. flet, an obsolete word of Teutonic origin, meaning the ground beneath the feet)

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 (Lat. forma)

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

family are practically always determinable by naked See also:

eye inspection or with the aid of a See also:

SIMPLE

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

part of sedimentary material which has been contact-altered, but they have also been regarded as having been See also:

DEVELOPED

developed by metamorphic See also:

ACTION

action out of biotite or other See also:

PRIMARY

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

sphene and epidote. Many of them are See also:

rich in hornblende and thus form transitions to amphibolites. See also:

PYROXENE

Pyroxene-gneisses are less frequent but occur in many parts of both hemispheres. The " See also:

CHARNOCKITE

charnockite " series are very closely allied to the pyroxene-gneisses. See also:

HYPERSTHENE

Hypersthene and See also:

SCAPOLITE (Gr. O'Kairos, rod, )tLOoc, stone)

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

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

geological See also:

DISTRIBUTION (Lat. distribuere, to deal out)

distribution of gneissose rocks. All the factors which are required for their See also:

PRODUCTION (Lat. productionem, from producere, to pro-duce)

production, See also:

HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

heat, See also:

MOVEMENT

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 [WILLIAM GRATTAN] TYRONE (1797-1841)

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

recent times. Hence, though most gneisses are Archean, all gneisses are not necessarily so. (J. S.

End of Article: GNEISS

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