Online Encyclopedia

Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.

VEINS

Online Encyclopedia
Originally appearing in Volume V27, Page 973 of the 1911 Encyclopedia Britannica.
» Make a correction to this article.
» Add information or comments to this article.
Spread the word: del.icio.us del.icio.us it!

VEINS , in See also:

geology, masses of See also:rock which occupy fissures in other rocks. They may have originated in many different ways and See also:present a See also:great variety of forms and structures. We may classify them in three See also:groups: (i.) veins of igneous rock, (ii.) of sedimentary, and (iii.) of minerals deposited by See also:water or by gases. Veins of igneous rock are practically the same as dikes; yet a distinction is sometimes made that dikes are narrow, often straight-walled and run for considerable distances, while veins are irregular, discontinuous and of• limited extent. Where See also:granite invades sedimentary or metamorphic rocks it very commonly emits vast See also:numbers of dikes. The margin Of the granite is full of blocks of all sizes, so that it is often impossible to say where the solid granite ends and the fringe of veins begins. An intrusion plexus of this sort seldom extends for more than a few See also:hundred yards; many granites, on the other See also:hand, have See also:sharp and well-defined margins and send few veins into the See also:country rock. In plutonic rock areas veining is also very See also:common. Great intrusive masses have not as a See also:rule been injected in one See also:stage but have been slowly enlarged by See also:gradual or repeated inflows, and often the earliest portions had consolidated before the last were introduced. Very frequently the' older rocks are of a different See also:character, being usually more basic than those which succeed them, and this makes the veining more obvious. For instance, it is common to find See also:peridotite traversed by many veins of See also:gabbro, or See also:diorite injected with numerous veins of granite, though in either See also:case the rocks are See also:part of one plutonic See also:boss or See also:laccolite. The crystalline structure of the vein-rock and the surrounding See also:mass is usually quite similar and there may be no See also:fine-grained edges to the veins; these facts establish that the older mass though solid had not yet cooled down, so that the veining is directly connected with the injection See also:process and the two rocks have been derived from the same source, but one is slightly later than the other.

Among the Laurentian or Lewisian gneisses, which; resemble granites, diorites and gabbros in See also:

composition, but have a banded or foliated structure, veining of this type is almost universal. The veins are of all sizes and of very irregular shape.. Frequently they run along the foliation of the See also:gneiss, but often also they See also:cross it. obliquely or at right angles. Such gneisses were produced by the injection of a partly differentiated and consequently non-homogeneous magma, by successive stages, under a rock crust which was in See also:movement or was subjected to intermittent pressures during consolidation. In certain cases he new material introduced into the rock by these veins bulks almost as largely as the See also:original substance. A shale, See also:slate or See also:phyllite is sometimes so filled with threads of granite that its composition and See also:appearance are completely altered. Thin palethreads of See also:quartz and See also:felspar, not more than a tenth of an See also:inch in thickness may be seen following the bedding planes, or the cleavage and sometimes also the slip•cleavage. The distance between the veins may be no greater than the breadth of the veins themselves, and. thus a striped or banded rock is produced, resembling a gniess but of dual origin, a mixed rock which is described properly as a composite " or " synthetic" gneiss. The See also:French geologists who first insisted on the importance of this See also:group of rocks have called the process lit See also:par lit (See also:bed-by-bed) injection. The best examples of this in See also:Britain are to be found around the granites of See also:Mull and See also:northern Su*herlandshire. The rocks invaded by granite in this manner often show intense contact alteration and are tc a large extent recrystallized. The See also:short irregular veins which commonly occur within areas of granite, diorite, gabbro and other plutonic rocks are often much more coarsely crystalline than the. rock around them.

This is • no doubt partly due to the high temperature of the whole complex and to slow See also:

crystallization, but it may also be ascribed to the See also:action of vapours dissolved in the magma and gradually released as it solidifies. Such coarse-grained igneous rocks are celled pegmatites (q.v, ). It is clear that they are not purely igneous but are partly pneumatblytic. With .the pegmatites we may class the fine-grained See also:acid veins (aplites) which are found not only i i granites but also 4n many diabases. They occur in irregular streaks or See also:gas See also:long branching well-defined veins, and are usually more See also:rich in quartz and felspar than the surrounding rock. Formerly they were often described as contemporaneous or as segregation veins; but no vein can be in strict aceur`dcy contemporaneous with the rock which it intersects, and many of them dive See also:evidence of having been intruded into their present situation, since their minerals are so arrakged as to show flexion structure. But they are always intimately connected, as their See also:mineral composition indicates, with the hock mass in which they See also:lie, and they represent merely the last part of the magma to consolidate. The fissures they occupy are presumably due to contraction, seeing that they are not accompanied by displacement, brecciation or faulting. Veins of sedimentary rock are few and of little importance: They occur where sediment has gathered in cavities of other rocks. See also:Lava streams, for example, when they cool, become split up into irregular blocks, and in the crevices between these ashes, See also:sand and See also:clay will See also:settle. Submarine lavas are often traversed by great numbers of thin veins of See also:sandstone, and a similar phenomenon may also be noted in the See also:tuff of submarine necks or other ash beds. Cracks in See also:limestone and See also:dolomite are widened by the solvent action of percolating See also:waters and may be filled with See also:gravel, See also:soil, clay and sand.

In the Carbdniferous Limestone, for instance, veins of bedded sandstone sometimes pass down from overlying Triassic deposits. The upper; See also:

surface of the See also:chalk in the See also:south of See also:England has frequently many deep See also:funnel-shaped pipes which are occupied by See also:Tertiary or See also:recent accnatttlAlions. The tdiird group of veins, namely, those which have been filled by deposits from See also:solution in water or in vapours, is of the greatest importance. as including a See also:veil y.large number of jnineral veins and ore-bodies: They are also the source of the great See also:majority of the finely ";cry. talfized.specimens of minerals. The deposfti9x pf minerals on the walls of fissures by a;proeess of sublimation See also:play be observed at any active See also:volcano. The cracks in the upper. rt of lava flows are often lined by;crystals of heal-ammoniac, sodtrm chloride, ferric chloride and•otb.r volatile sub-stances By oxtdationcf the See also:iron chloride See also:bright scales of See also:haematite (ferric See also:oxide) arise; splphurous acid and sulphuretted ;See also:hydrogen, given out as -gases, react on one another, producing *yellow encrustations of See also:sulphur; and See also:copper oxide (tenorite) and a great varietyy of. other' minerals (See also:alum, iron sulphate, See also:realgar, berates and fluoride) are found about fumaroles of See also:Vesuvius and other volcanoes. Most veins however, are not of superficiirl origin but have been formed at some See also:depth. The See also:heat given out by messes of rocks which were injeced in a molten See also:state is no doubt sufficiently high to volatilize many minerals. The pressure, however,, also must be taken into See also:account, as it tends to retain these substances in a liquid See also:condition. Water vapour is always the most abundant gas in a volcanic magma, and next to it are carbonic acid, sulphurous acid; sulphuretted hydrogen and hydrochloric acid. The See also:physical condition of the substances passing putwards from an igneous mass through fissures in the superincumbent rocks will depend on the nature of the substances, on the temperature and the pressure. Near the granite the heat is so great, at first at any See also:rate, that gaseous materials must greatly preponderate; but farther away many of them will be condensed and hot aqueous solutions of complex composition will fill the cracks. Veins deposited by the action of gases and vapours are said to be of " pneumatolytic," origin; where hot aqueous solutions have been the See also:principal agency in their formation they are " h datogenetic." It is often very difficult to ascertain to which of these classes a mineral vein belongs, especially as we are in See also:ignorance of the behaviour of many substances at high temperatures and under great pressures.

The veins which yield See also:

tin-ores in See also:Cornwall and in most other tin-producing countries are generally regarded as typical pneumatolytic deposits. Tin forms a volatile fluoride which may be decomposed by water, forming tin oxide, the See also:fluorine passing' into hydrofluoric acid which may See also:act as a catalytic See also:agent or See also:carrier by again combining with tin. Around tin-bearing veins and in the material which fills them there are usually' many minerals containing fluorine, such as See also:topaz, fluor-spar and See also:white See also:mica. Some borates too are volatile at high temperatures, and minerals containing See also:boron (especially See also:tourmaline) are very common in tin veins. Also since ore deposits of this character are found nearly invariably in granite or in the rocks which have been invaded by granite there is See also:good See also:reason to hold that fluoric and boric gases were important agents in the See also:production of tin veins. It is not necessary, however, to believe that a1 the materials which are found in these veins were introduced as vapours, for as the temperature sank currents of hot water would follow which would fill up any cavities. The tin veins of Cornwall often contain copper ores in their upper parts and at greater distances from the granite, a fact which indicates that the copper salts were deposited from solution at See also:lower temperatures than the tin ores. A very large number of important ore deposits have been laid down by hot waters emanating from deep-seated intrusive masses. Nearly all the principal goldfields (except gravels or placers) are in districts where igneous dikes, veins and sills abound, and it is often perfectly clear that the introduction of the See also:gold ores is intimately connected with the intrusive masses. The Witwatersrand deposits, although by many considered to be old auriferous gravel, have been regarded as owing their value to gold deposited from vapours emanating from certain of the dikes which See also:traverse the See also:banket rock or See also:conglomerate. The importance of these hot ascending currents of water, proceeding from eruptive magmas, has been fully recognized, and is now probably the most widely accepted theory of the See also:genesis of mineral veins. The water falling on the See also:earth's surface will to a large extent percolate downwards into the rocks, and it will dissolve mineral matters, especially at the greater depths, owing to the increased temperature and pressure; conversely, as it ascends it will See also:lay down deposits or veins.

This is the theory of " lateral secretion," at one See also:

time in great favour, but now regarded as of less importance. Ferruginous waters on passing through limestone rocks may de-posit their iron as haematite or siderite, removing a proportionate amount of See also:lime, and in this way great bodies of ironstone have been formed, as in See also:Cumberland and See also:Yorkshire, partly along the bedding of the limestone but also in veins, pockets and irregular masses. Many See also:lead and See also:zinc veins probably belong also to this class. By See also:analysis it has been proved that in nearly all the common rocks there exist very See also:minute quantities of such metals as gold, See also:silver, lead, copper, zinc. If these can be extracted in solution in water they might conceivably be deposited subsequently in fissures in the rocks. Controversy has raged between opposing See also:schools of geologists, one considering that most mineral veins owe their existence to currents of hot water ascending from deep-seated igneous rocks, and the other that the metals were derived from the country rocks of the veins and were extracted from them by See also:cold descending currents of water. There are cases which can be explained on one of these hypotheses only, and sufficiently establish that both of them are valid; but the See also:general See also:opinion at the present time is in favour of the first of these explanations as the most general. The fissures in which veins have been deposited owe their origin to a variety of causes. Many of them are lines of See also:fault, the walls of which have been displaced before the introduction of the vein minerals. Others seem to be of the same nature as See also:joints, and are due either to contraction of the rocks on solidification, to folding or to See also:earthquake shocks. In the vicinity of intrusive masses many fissures have been produced by the contraction of rock masses which had been greatly heated and then slowly cooled. Veins often occur in groups or systems, which have a parallel trend and may some-times be followed for many See also:miles.

The larger veins may See also:

branch and the branches sometimes unite after a time, enclosing masses of country rock or " horses." Cross-courses are fissures which inter-See also:sect the lodes; they are often barren, and at other times carry an entirely different See also:suite of minerals from those of the mineral veins. A See also:peculiar group of veins has been described from the See also:Bendigo See also:district of See also:Australia; they are See also:saddle-shaped and in transverse See also:section resemble an inverted U. The beds in which they occur are folded sharply into See also:arches and troughs, and in folding they have separated at the crests of the arches, leaving hollows which were subsequently filled up with ore. The minerals occurring in the veins are sometimes classified as " ores " and " See also:gangue ": the former being those which are of value while the others are unprofitable. The commonest of the gangue minerals are quartz, See also:calcite, See also:barytes and fluor-spar. Usually a large number of minerals occurs in each vein, and the natural association or " paragenesis " of certain minerals which frequentlyare found together is a See also:practical See also:guide of much value to the engineer and prospector. A definite sequence in the See also:order of See also:deposit of the constituent minerals can often be recognized, the earlier being situated on the walls of the fissures or enclosed and surrounded by the later, and the microscopic study of veinstone shows that they have often a complicated See also:history. Many types of structure are met with in veinstones and vein deposits. Some are structureless, homogeneous or massive, like the quartz veins which are often found in districts composed of slate or phyllite. Others are banded, with sheets of deposit, each consisting of one mineral, usually parallel to the walls of the lode. These veins are often symmetrical, with corresponding layers following one another inwards from the walls on each See also:side. The veinstones are frequently crushed either by faulting or by irregular movements of the walls, and in such cases the veinstones have a shattered or brecciated appearance.

If the crushing took See also:

place while the ore deposits were still being introduced, the broken rock is often cemented together into a compact mass. Rounded masses of rock or of veinstone are often met with, looking exactly like pebbles, but they are analogous to crush-conglomerates, as the fragments have been shaped by the movements of the walls of the vein. Frequently these movements have reopened a fissure which had been filled up, and a new vein is subsequently formed alongside of the old one; this process may be repeated several times. The mineral-bearing solutions may exert a powerful See also:influence on the walls of the veins, removing certain constituents and depositing others; in this way the walls of the vein become See also:ill defined. The commonest See also:change of this See also:kind is silicification, and rocks of many different kinds, such as slate, limestone, See also:andesite and See also:felsite, are often completely replaced by quartz in the vicinity of mineral veins which have a quartzose gangue. Tin veins in granite and slate may be surrounded by a See also:zone of rock which has been impregnated with cassiterite and is See also:worth working for the See also:metal. These changes are of a " metasomatic " type involving replacement of the original rock-substance by introduced materials. Many of the best examples of this are furnished by limestone, which is one of the rocks most easily affected by percolating solutions. The distinction between mineral veins and other veins is to a large extent artificial. With improvement of methods of See also:mining and extraction deposits formerly unprofitable become payable, and in all cases veins vary considerably in the amount of ore they carry. The rich parts are sometimes called shoots or bonanzas, while the barren portions are often See also:left See also:standing in the mine. Near the ground surface the veinstones become oxidized and the metallic minerals are represented by oxides, See also:carbonates, hydrates, or in the case of gold and silver veins they may be rich in the metals them-selves.

Below the zone where oxidizing surface-waters percolate a different See also:

series of minerals occurs, such as sulphides, arsenides and tellurides. If the ores are insoluble they will tend to be concentrated in the upper part of the vein rock, which may be greatly enriched in this way. Pyritic veins are changed to rusty-looking masses, " gossans," owing to the oxidation of the iron at the surface. Though instances are known of veins which come to an end when followed downwards, it seems probable that the majority of veins descend to great depths, and there is little reason to believe that they become less rich in the heavy metals. (J. S.

End of Article: VEINS

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to 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.

[back]
VEIL (O.Fr. veile, mod. voile, from Lat. velum, clo... 		[next]
VEIT, PHILIPP (1793-1877)

Site © 2006 - Net Industries