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green rocks, some of which are true gabbros, while others are serpentines. The gabbros are the plutonic or deep-seated representatives of the dolerites, basalts and diabases (also of some varieties of See also:

ANDESITE

andesite) with which they agree closely in See also:

MINERAL

mineral See also:

composition, but not in See also:

minute structure. Of their minerals felspar is usually the most abundant; and is principally See also:

LABRADORITE, or LABRADOR SPAR

labradorite and See also:

BYTOWNITE

bytownite, though See also:

ANORTHITE

anorthite occurs in some, while See also:

OLIGOCLASE

oligoclase and See also:

ORTHOCLASE

orthoclase have been found in others. The felspar is sometimes very clear and fresh, its crystals being for the most See also:

PART

part See also:

SHORT, FRANCIS JOB (1857– )

short and broad, with rather irregular or rounded outlines. See also:

ALBITE

Albite twinning is very frequent, but in these rocks it is often accompanied by pericline twinning by which the broad or narrow albite plates are cut transversely by many thin, See also:

bright and dark bars as seen in polarized See also:

LIGHT

light. Equally characteristic of the gabbros is the alteration of the felspars to cloudy, semi-opaque masses of saussurite. These are compact, tough, devoid of cleavage, and have a waxy lustre and usually a greenish-See also:

white See also:

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

colour. When this substance can be resolved by the See also:

MICROSCOPE (Gr. µucp6s, small, asenreiv, to %view)

microscope it proves to consist usually of See also:

ZOISITE

zoisite or See also:

EPIDOTE

epidote, with See also:

garnet and albite, but mixed with it are also See also:

CHLORITE

chlorite, See also:

AMPHIBOLE

amphibole, See also:

SERPENTINE

serpentine, See also:

PREHNITE

prehnite, sericite and other minerals. The augite is usually See also:

brown, but greenish, See also:

VIOLET

violet and colourless varieties may occur. Hypersthene, when See also:

PRESENT

present, is often strikingly pleochroic in See also:

COLOURS, MILITARY

colours varying from See also:

PINK

pink to bright green. It weathers readily to platy-pseudomorphs of bastite which are soft and yield See also:

low polarization colours.

The olivine is colourless in itself, but in most cases is altered to green or yellow serpentine, often with bands of dark See also:

MAGNETITE

magnetite granules along its cleavages and cracks. Hornblende when See also:

PRIMARY

primary is often brown, and may surround augite or be perthitically intergrown with it'; See also:

ORIGINAL

original green hornblende probably occurs also, though it is more frequently secondary. Dark-brown See also:

BIOTITE

biotite, although by no means an important constituent of these rocks, occurs in many of them. See also:

QUARTZ

Quartz is rare, but is occasionally seen intergrown with felspar as micropegmatite. Among the See also:

ACCESSORY

accessory minerals may be mentioned See also:

APATITE

apatite, magnetite, See also:

ILMENITE

ilmenite, picotite and garnet. A See also:

PECULIAR

peculiar feature, repeated so constantly in many of the minerals of these rocks as to be almost typical of them, is the occurrence of small See also:

black or dark brown enclosures often regularly arranged parallel to certain crystallographic planes. Reflection of light from the surfaces of these minute enclosures produces a shimmering or See also:

SCHILLER, JOHANN CHRISTOPH FRIEDRICH VON (1759-18o5)

Schiller. In augite or hypersthene the effect is that the See also:

SURFACE

surface of the mineral has a bronzy sub-metallic See also:

APPEARANCE (from Lat. apparere, to appear)

appearance, and polished plates seen at a definite See also:

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle yield a bright coppery-red reflection, but polished sections of the felspars may exhibit a brilliant See also:

PLAY

play of colours, as is well seen in the Labrador spar, which is used as an ornamental or semi-See also:

PRECIOUS (O. Fr. precios, mod. precieux, Lat. pretiosus, of high value or price, pretium)

precious See also:

stone. In olivine the black enclosures are not thin laminae, but branching growths resembling pieces of See also:

MOSS

moss. The phenomenon is known as schillerization "; its origin has been much discussed, some holding that it is secondary, while others regard these enclosures as original. In many gabbros there is a tendency to a centric arrangement of the minerals, the first crystallized forming nuclei around which the others grow. Thus magnetite, apatite and picotite, with olivine, may be enclosed in augite, hornblende, and hypersthene, sometimes with a later growth of biotite, while the felspars occupy the interspaces between the clusters of ferromagnesian minerals.

In some cases there are See also:

BORDERS, THE

borders around olivine consisting of fibrous hornblende or See also:

TREMOLITE

tremolite and rhombic See also:

PYROXENE

pyroxene (kelyphitic or ocellar structures); spinels and garnet may occur in this See also:

ZONE (Gr. 'WV77, a girdle, from 'wvvLvay to gird)

zone, and as it is See also:

DEVELOPED

developed most frequently where olivine is in contact with felspar it may be due to a chemical resorption at a See also:

LATE

late See also:

STAGE (Fr. 6/age; from Lat. stare, to stand)

stage in the solidification of the See also:

rock. In some gabbros and norites reaction rims of fibrous hornblende are found around both hypersthene and See also:

DIALLAGE

diallage where these are in contact with felspar. Typical orbicular structure such as characterizes some granites and diorites is rare in the gabbros, though it has been observed in a few instances in See also:

Norway, See also:

California, &c. In a very large number of the rocks of this group the plagioclase felspar has crystallized in large measure before the pyroxene, and is enveloped by it in ophitic manner exactly as occurs in the diabases. When these rocks become See also:

FINE

fine-grained they pass gradually into ophitic See also:

DIABASE

diabase and See also:

DOLERITE (from Gr. SoXepos, deceptive)

dolerite; only very rarely does olivine enclose felspar in this way. A fluxion structure or flow banding also can be observed in some of the rocks of this See also:

SERIES (a Latin word from serere, to join)

series, and is characterized by the occurrence of parallel sinuous bands of dark colour, See also:

rich in ferromagnesian minerals, and of lighter shades in which felspars predominate. These basic holocrystalline rocks See also:

FORM (Lat. forma)

form a large and numerous class which can be subdivided into many See also:

groups according to their mineral composition; if we take it that typical gabbro consists of plagioclase and augites or diallage, norite of plagioclase and hypersthene, and troctolite of plagioclase and olivine, we must add to these olivine-gabbro and olivinenorite in which that mineral occurs in addition to those enumerated above. Hornblende-gabbros are distinctly rare, except when the hornblende has been developed from pyroxene by pressure and shearing, but many rocks may be described as hornblende- or biotite-bearing gabbro and norite, when they contain these ingredients in addition to the normal minerals plagioclase, augite and hypersthene. We may recognize also quartz-gabbro and quartz-norite (containirg primary quartz or micropegmatite) and orthoclase-gabbro (with a little orthoclase). The name eucrite has been given to gabbros in which the felspar is mainly anorthite; many of them also contain hypersthene or See also:

ENSTATITE

enstatite and olivine, while allivalites are anorthite-olivine rocks in which the two minerals occur in nearly equal proportions; harrisites have preponderating olivine, anorthite felspar and a little pyroxene. In areas of gabbro there are often masses consisting nearly entirely of a single mineral, for example, felspar rocks (anorthosites), augite or hornblende rocks (pyroxenites and hornblendites) and olivine rocks (dunites or peridotites). Segregations of See also:

IRON [symbol Fe, atomic weight 55.85 (0=16)]

iron ores, such as ilmenite, usually with pyroxene.or olivine, occur in association with some gabbro and anorthosite masses.

Some gabbros are exceedingly coarse-grained and consist of individual crystals several inches in length; such a type often form dikes or See also:

VEINS

veins in serpentine or gabbro, and may be called gabbropegmatite. Very fine-grained gabbros, on the other See also:

hand, have been distinguished as beerbachites. Still more See also:

COMMON

common is the occurrence of sheared, foliated or schistose forms of gabbro. In these the minerals have a parallel arrangement, the felspars are often broken down by pressure into a See also:

MOSAIC (corresponding to Lat. opus musivum, from Gr. µovoeiov, an artificial grotto often decorated with mosaics; the word is only found in the sense of mosaic in late Greek, which generally uses k oXI yrlµa)

mosaic of irregular grains, while greenish fibrous or bladed amphibole takes the See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place of pyroxene and olivine. The diallage may be present as rounded or See also:

OVAL (Lat. ovum, egg)

oval crystals around which the crushed felspar has flowed (augen-gabbro); or the whole rock may have a well-foliated structure (hornblende-See also:

SCHISTS (Gr. vxq'ecv, to split)

schists and amphibolites). Very often a See also:

mass of normal gabbro with typical igneous See also:

CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

character passes at its margins or along localized zones into foliated rocks of this See also:

KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind, and every transition can be found between the different types. Some authors believe that the development of saussurite from felspar is also dependent on pressure rather than on weathering, and an analogous See also:

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change may affect the olivine, replacing it by See also:

TALC

talc, chlorite, actinolite and garnet. Rocks showing changes of the latter type have been described from See also:

SWITZERLAND

Switzerland under the name allalinites. Rocks of the gabbro group, though perhaps not so common nor occurring in so See also:

GREAT

great masses as granites, are exceedingly widespread. In Great See also:

BRITAIN (Gr. Hperavuml vi7vot, Bperravia; Lat. Britannia, rarely Britannia)

Britain, for example, there are areas of gabbro in See also:

SHETLAND, or ZETLAND

Shetland, See also:

ABERDEENSHIRE

Aberdeenshire, and other parts of the See also:

HIGHLANDS, THE

Highlands, See also:

AYRSHIRE

Ayrshire, the See also:

LIZARD (Lat. lacertal)

Lizard (See also:

CORNWALL

Cornwall), Carrock See also:

Fell (See also:

Cumberland) and St See also:

David's (See also:

WALES (Cymru, Gwalia, Cambria)

Wales). Most of these occur along with troctolites, norites, serpentine and See also:

PERIDOTITE

peridotite. In See also:

SKYE

Skye an interesting group of fresh olivinegabbros is found in the Cuillin Hills; here also peridotites occur and there are sills and dikes of olivine-dolerite, while a great series of basaltic lavas and ash beds marks the site of volcanic outbursts in See also:

early See also:

TERTIARY

Tertiary See also:

time.

In this See also:

case it is clearly seen that the gabbros are the deep-seated and slowly crystallized representatives of the basalts which were poured out at the surfaces, and the dolerites which consolidated in fissures. The older gabbros of Britain, such as those of the Lizard, Aberdeenshire and Ayrshire, are often more or less foliated and show a tendency to pass into hornblende-schists and amphibolites. In See also:

Germany gabbros are well known in the Harz Mountains, See also:

Saxony, the See also:

ODENWALD

Odenwald and the Black See also:

FOREST

Forest. Many outcrops of similar rocks have been traced in the See also:

NORTHERN

northern zones of the See also:

ALPS

Alps, often with serpentine and hornblende-schist. They occupy considerable tracts of See also:

COUNTRY (from the Mid. Eng. contre or contrie, and O. Fr. cuntree; Late Lat. contrata, showing the derivation from contra, opposite, over against, thus the tract of land which fronts the sight, cf. Ger. Gegend, neighbourhood)

country in Norway and See also:

Sweden, as for instance in the vicinity of See also:

BERGEN

Bergen. The See also:

PYRENEES [Span. Pirineos, Fr. Pyrenees]

Pyrenees, Ligurian Alps, See also:

DAUPHINE

Dauphine and See also:

Tuscany are other See also:

EUROPEAN

European localities for gabbro. In See also:

CANADA

Canada great portions of the eastern portion of the Dominion are formed of gabbros, norite, anorthosite and allied rock types. In the See also:

UNITED

United States gabbros and norites occur near See also:

Baltimore and near See also:

PEEKSKILL

Peekskill on the See also:

Hudson See also:

RIVER

river. As a See also:

RULE

rule each of these occurrences contains a diversity of petrographical types, which appear also in certain of the others; but there is often a well-marked individuality about the rocks of the various districts in which gabbros are found. From an economic standpoint gabbros are not of great importance. They are used locally for See also:

BUILDING

building and for road-See also:

metal, but are too dark in colour, too tough and difficult to See also:

DRESS (from the Fr. dresser, to set out, arrange, formed from Lat. directus, arranged, dirigere, to direct, arrange)

dress, to be popular as building stones, and, though occasionally polished, are not to be compared for beauty with the serpentines and the granites'. Segregations of iron ores are found in connexion with many of them(Norway and Sweden) and are sometimes See also:

MINED

mined as See also:

SOURCES

sources of the metal.

Chemically the gabbros are typical rocks of the basic subdivision and show the characters of that group in the clearest way. They have low See also:

SILICA

silica, much iron and See also:

MAGNESIA

magnesia, and the abundance of See also:

lime distinguishes them in a marked See also:

FASHION (adapted from Fr. facon, agon, Lat. factio, making, facere, to do or make)

fashion from both the granites and the peridotites. A few analyses of well-known gabbros are cited here. I. Gabbro, Radanthal, See also:

HARZBURG

Harzburg; II. Gabbro, Penig, Saxony; III. Troctolite, Coverack, Cornwall ; IV. Anorthosite, mouth of the See also:

Seine river, See also:

BAD, BCD

Bad See also:

VERMILION

Vermilion See also:

lake, See also:

Ontario, Canada. (J. S. F.) .

End of Article: GABBRO

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