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VEINS—THEIE NATUEE AND OEIGIN.
VEINS AND DYKES, HOW OCCASIONED—METALLIFEROUS VEINS, MOST ABUNDANT IN THE PRIMARY ROCKS—REASON OF THIS ABUNDANCE—GENERAL CHARACTERISTICS OF VEINS—VARYING NATURE OF THE MINERAL SUBSTANCES BY WHICH VEINS ARE MAINLY FILLED—MODE IN WHICH THESE VEIN-STUFFS ARE AGGREGATED —RELATIONS OF THE ORES TO THE VEIN-STUFFS—DIRECTION OF VEINS IN CERTAIN LOCALITIES—RIGHT-RUNNING AND CROSS VEINS —THEORIES OF FORMATION AND FILLING—INFILTRATION AND DEPOSITION OF CHEMICAL SOLUTIONS — EFFECTS OF THERMAL AGENCY—ELECTRIC AND OTHER KINDRED CURRENTS—RELATIONS TO CENTRES OF IGNEOUS ACTION—IMPORTANCE OF METALLIFEROUS VEINS—CONTRAST BETWEEN THE MINERAL AND AGRICULTURAL VALUE OF PRIMARY DISTRICTS—MINES AND STREAMWORKS—MINING INDUSTRY, AND IMPORTANCE OF SOUND GEOLOGICAL DEDUCTION.
It has been stated in the preceding Sketch, that of all the rocks in the earth's crust, the primary are those most abundantly traversed by veins. As these veins are the great repositories of the metallic ores, it may be useful at this stage briefly to explain what veins are, how they occur, and what the general character of their contents. This information may lead to a better comprehension of much that will be subsequently stated, at the same time that it is valuable knowledge of itself, and belongs to one of the most interesting departments of modern geology.
It was mentioned under Vulcanicity, that in all volcanic areas the solid crust was more or less rent and fissured— these rents either radiating from some centre of eruption, or running parallel to each other according to the most prevalent direction of the earthquake convulsions. These fractures will vary, of course, from mere cracks to yawning chasms many feet in width, and will descend at all inclinations—some sloping downwards at a low angle, and others sinking perpendicularly, or nearly so. Now it requires no great effort of the understanding to perceive that these fissures, in course of time, will be filled either by matter washed in from the earth's surface, or by volcanic and mineral substances injected from the interior. Such rents in the stratified rocks, when thus filled up by lava, by greenstone, or by basalt, are generally known as "dykes," the igneous matter rising up like a wall through the strata on either side. On the other hand, when they are filled by sparry or crystalline minerals, and these intermingled more or less with metallic ores—the slow and gradual depositions of chemical agency—they are usually distinguished as "Veins," from their traversing and ramifying through the crust like the veins through the animal system. But the veins that were formed at one period may be cut through or crossed by others of a later era, and thus in many districts there is a network, as it were, of veins, crossing and intercrossing in a very complicated manner. As might be expected, too, the original veins may contain one kind of mineral or metal, and the cross-veins another kind, and hence the greater richness, as well as complexity, of many metalliferous regions.
Understanding, then, that " dykes" consist of unproductive rock-matter, and that " veins" are always less or more metalliferous, it may be stated as a fact, that the latter occur most abundantly, and naturally so, in the primary formations. These are the rocks that have suffered most from igneous convulsions, and these also are the rocks among which metamorphism and chemical agency have had longest time to bring about internal change and fill with crystalline and metalliferous depositions. There are, no doubt, productive veins in later formations, such as the lead and silver bearing veins of the Carboniferous limestone; but these by no means occur in the same richness and variety as those of the primary strata. Wherever, then, we have extensive developments of primary rocks and mountains, as in Wales and Cornwall, Scandinavia, the Ourals, the Andes and Mexican Sierras, there also we may expect a corresponding development of metalliferous veins —gold, silver, tin, copper, and the like—of varying age and richness, according to certain laws the order and governance of which geology is yet unable to indicate. The formation and accumulation of rocks is in most instances a slow and gradual process, but the-segregation and deposition of metalliferous matter is still slower, and thus we may look upon the veins of the primary strata as of high antiquity, though necessarily younger than the rocks they traverse. Occurring most abundantly in the older formations, and very rarely in those of secondary or tertiary date, it is to the veins and veinstones of primary regions that the following remarks will be more especially directed.
Denning a vein as a rent or fissure in the earth's crust which has been subsequently filled up by infiltrations of mineral and metallic matter, it must be obvious that veins will be of various widths and of various inclinations. Productive veins seldom exceed a few feet in width, and it is rare to find them beyond fifteen or twenty; but their inclinations are at all angles—some descending almost perpendicularly, and others sloping downwards by very easy stages. The bounding rocks on either side form the cheeks or walls of a vein; the mineral matter of which it is composed, the vein-stuff, matrix, or gangue; and the metallic ore is distributed through the matrix in ribs, pockets, nests, strings, and plates, according to the manner and abundance of its occurrence. The vein-stuff is usually arranged in layers from the walls inwards, the centre being generally occupied by a rib of ore, though not unfrequently hollow and lined with crystals. The whole matrix has thus a striped or veined appearance, the stripes running up and down or parallel to the cheeks or containing walls. It must be obvious from this description that a vein is something very distinct from the rocks through which it passes. If it pass through igneous rocks, its stripes and colours contrast very strongly with the dark uniform hues of these masses; if through sedimentary rocks, its upward and downward course through their strata at once arrests the attention; and, generally speaking, its sparry or crystalline texture is sufficient to define its thickness and direction. Passing from below upwards, and frequently ramifying, crossing, and intercrossing in many directions, they look indeed like the veins in vegetable and animal structures, and hence their appropriate and expressive designation.
The sparry matter which forms the bulk of the veinstone or matrix consists for the most part of quartz, carbonate and sulphate of lime, carbonate and sulphate of baryta, or of alternations and admixtures of these—the ore occupying a subordinate part in ribs, strings, nests, and pockets. These vein-stuffs seem to have been deposited first on the cheeks, and then coating after coating towards the centre, which is either solid like the rest of the matrix, or hollow, as if there had been a deficiency of filling matter; and in such cases the cavity is lined with crystals shooting and pointing inwards. Having had room to assume their independent forms, the crystals in these cavities are often of great beauty, and it is usually from such veinspaces that the mineralogist obtains his rarest treasures.
In some instances the vein-stuff consists of a single substance in repeated coatings, such as quartz or carbonate of lime; in other instances it is made up of alternating layers of two substances, Such as lime and baryta; and in many cases it consists of a seemingly capricious admixture of several ingredients. As with the vein-stuffs so with the enclosed metallic ores — some veins containing only one metal, others two or more metals, and in such cases there is usually a curious and persistent connection, as lead with silver, copper with tin, iron with manganese, and gold with platinum. Besides this curious connection of metal with metal, there is often an observed relation between certain ores and certain vein-stuffs, as gold in quartz, lead in carbonates and fluates of lime, &c.; and these relations, when carefully noted, are often of great practical value to the mineral explorer. What has caused these curious alternations of vein-stuffs and connections of certain metals science cannot in the mean time determine, but it observes the modes in which they occur, and this, when accurately done, is always a step towards the solution. Whatever the causes, they have operated not always in filling the veins and fissures merely, but often in impregnating the adjacent rocks through which the veins pass, and thus the cheeks or walls are occasionally worked for the strings and plates of the ores they contain.
But though unable to explain the relations of certain ores to certain vein-stuffs, geology has amassed a great deal of observation on the subject, as well as on the prevalent direction or strike which veins take in certain localities. The ascertaining of this direction is all-important; for while the main veins of a district containing one kind of metal are found to strike always in one direction— easterly and westerly, for instance—the secondary or cross veins running northerly and southerly are almost certain to