been the principal agency in their formation they are “hydatogenetic.” It is often very difficult to ascertain to which of these classes a mineral vein belongs, especially as we are in ignorance of the behaviour of many substances at high temperatures and under great pressures.
The veins which yield tin-ores in 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 fluorine passing into hydrofluoric acid which may act as a catalytic agent or 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 topaz, fluor-spar and white mica. Some borates too are volatile at high temperatures, and minerals containing boron (especially 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 good reason to hold that fluoric and boric gases were important agents in the production of tin veins. It is not necessary, however, to believe that all 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 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 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 traverse the banket rock or 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 genesis of mineral veins.
The water falling on the 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 lay down deposits or veins. This is the theory of “lateral secretion,” at one time in great favour, but now regarded as of less importance. Ferruginous waters on passing through limestone rocks may deposit their iron as haematite or siderite, removing a proportionate amount of lime, and in this way great bodies of ironstone have been formed, as in Cumberland and Yorkshire, partly along the bedding of the limestone but also in veins, pockets and irregular masses. Many lead and zinc veins probably belong also to this class. By analysis it has been proved that in nearly all the common rocks there exist very minute quantities of such metals as gold, 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 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 cold descending currents of water. There are cases which can be explained on one of these by hypotheses only, and sufficiently establish that both of them are valid; but the general 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 fault, the walls of which have been displaced before the introduction of the vein minerals. Others seem to be of the same nature as joints, and are due either to contraction of the rocks on solidification, to folding or to 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 sometimes be followed for many miles. The larger veins may branch and the branches sometimes unite after a time, enclosing masses of country rock or “horses.” Cross-courses are fissures which intersect the lodes; they are often barren, and at other times carry an entirely different suite of minerals from those of the mineral veins. A peculiar group of veins has been described from the Bendigo district of Australia; they are saddle-shaped and in transverse section resemble an inverted U. The beds in which the occur are folded sharply into 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 “gangue”: the former being those which are of value while the others are unprofitable. The commonest of the gangue minerals are quartz, calcite, barytes and fluor-spar. Usually a large number of minerals occurs in each vein, an the natural association or “paragenesis” of certain minerals which frequently are found together is a practical guide of much value to the engineer and prospector. A definite sequence in the order of 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 vein stone shows that they have often a complicated history.
Many types of structure are met with in vein stones 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 side.
The vein stones are frequently crushed either by faulting or by irregular movements of the walls, and in such cases the vein stones have a shattered or brecciated appearance. If the crushing took 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 vein stone 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 influence on the walls of the veins, removing certain constituents and depositing others; in this way the walls of the vein become ill defined. The commonest change of this kind is silicification, and rocks of many different kinds, such as slate, limestone, andesite and felsite, are often completely replaced by quartz in the vicinity of mineral veins which have a quartzose gangue. Tin veins in granite and slate mai; be surrounded by a zone of rock which has been impregnated wit cassiterite and is worth working for the 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 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 shcots or bonanzas, while the barren portions are often left standing in the mine. Near the ground surface the vein stones become oxidized and the metallic minerals are represented by oxides, carbonates, hydrates, or in the case of gold and silver veins they may be rich in the metals themselves. Below the zone where oxidizing surface-waters percolate a different 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. F.)
VEIT, PHILIPP (1793–1877), German painter, one of the
leaders of the German romantic school, was born in Berlin.
Having received his first art education in Dresden and Vienna,
he was strongly influenced by, and joined the group of, the
Nazarenes in Rome, where he worked for some years before
taking up his abode in Frankfort. In this city, where his
most important works are preserved at the Staedel Institute,
he was active front 1830 to 1843, as director of the art collections and as professor of painting. From 1853 to his death in 1877 he held the post of director of the municipal gallery at Mayence. Like his fellow-Nazarenes he was more draughtsman than painter, and though his sense of colour was stronger than that of Overbeck or Cornelius, his works are generally more of the nature of coloured cartoons than of paintings in the modern sense. His principal work is the large fresco of "The Introduction of Christianity into Germany by St Boniface," at the Staedel Institute in Frankfort. In the cathedral of that city is his "Assumption," whilst the Berlin National Gallery has his painting of "The Two Marys at the Sepulchre." To Veit is due the credit of having been the first to revive the almost forgotten technique of fresco painting.
See Kunst, Künstler und Kunstwerke, by Valentin Veit.
VEITCH, JOHN (1829–1894), Scottish poet, philosopher, and historian of the Scottish border, son of a Peninsular veteran, was born at Peebles on the 24th of October 1829, and educated at Edinburgh University. He was assistant lecturer successively
