Mineral reins are sheets of metalliferous matter, which are divided into three principal varieties—gash veins, segregated veins, and fissure veins. Gash veins are those which have been formed in fissures that have resulted from the shrinking of the rock, and are limited to one rock, generally to one bed. Examples are seen in the leild mines of the upper Mississippi, where the ore is confined to the Galena limestone, a lower Silurian formation. It usually occurs in vertical fissures of little depth, but sometimes in hori zontal fissures, often opening into caves or chambers lined with ore; indeed, gash veins are often the commencements of cave formations. Segregated veins are those which are interposed between the strata, and always occur in metamorphic rocks, metamor phism being the cause of the segregation by the separation of the metalliferous materials from the masses of adjacent strata. They have not the banded character of the fissure veins (to be described), and are generally composed of quartz, often rich in gold. All the granitoid rocks of the Alleghanies are of this character, and contain more or less gold. Iron, and also copper, and less frequently nickel, are common associates. Fis sure veins, true veins, or lodes, are formed in fissures which have been produced by volcanic or earthquake action. The displacements caused by these forces result in the formation of fissures because of the inability of the strata to return to their former rela tional position from the interposition of wedges of rock or other eauses. The subsequent filling of the fissure by metalliferous material forms the vein or lode. As a consequence of the mode of formation, a fissure-vein is usually of unequal thickness, having the form of a wedge. They usually send out minor fissures, generally at acute angles, which are called branches, and sometimes feeders. The horizontal direction of a vein is called its strike or course, and is expressed by degrees of the quadrant in relation to points of the compass. The vertical angle which it makes with the horizon is called the dip. Geo logical disturbances are a frequent cause of displacement of parts of veins, forming what are called faults. The ores contained in fissure-veins are various, such as silver, copper, lead, tin, zinc, anthnony, and other metals. Gold is less common than in segregated veins, and it is usually only worked in them as a side product. Silver is the most valu able constituent, and these veins constitute the great silver repositories of the world. The Comstock lode and various others in Nevada are examples. Various theories have been advanced to account for the filling of mineral veins. The earliest was the plutonic theory, which supposed that the materials were injected into the fissures in a state of fusion. An irresistible objection to this theory is that such a method would have neces sitated the production of alloys to a much greater extent than is found to obtain. A
later theory regarded the formation as the result of aqueous deposition, or sedimentation, in the manner of limestone and other sedimentary rocks. An overwhelming objection to this theory is the fact that veins are not horizontally stratified, but the materials are often deposited in vertical positions against the walls of the fissures. The theory of lateral secretion or transfusion has been proposed, which supposes that the contents of the fissures were derived by percolation through the walls of the veins from the adjacent rocks; but a fatal objection to this theory is the fact that the composition of a mineral vein is often the same throukhout its extent, or in passing through various strata, whereas it ought to vary if the theory were true. _Again, two veins of dissimilar consti tution often traverse the same stratum adjacent to each other. This theory demands that they should be alike. The chemical precipitation theory regards the deposition as due to precipitation from superheated solutions under g,reat pressure. These solutions, coming from subterranean sources, part with heat in pa,ssing into the fissures. The deposits m. 4.e by thermal springs are tustancecl as affording illustrations of this mode of production. Water containing salts of various kinds is capable, when under great pressure and at a high temperature, of dissolving most minerals; and if it came in contact with snide acid charged with sulphur, many metals with which it came in contact in its pass age through the fissures would be reduced to sulphides and deposited on cooling. Illustra tions embracing the action of solutions of various saline and other bodies are carried to a vieater or less extent in systematic works. Most mineral veins are more or less decom posed when situated at or near the surface, and, indeed, this condition usualh extends dominwards to the permanent wat,er level, below which the ore is usually in us original state, which is, generally, a sulphide. In Cornwall the decomposed portion of a mineral vein is called a gossan, and this term is generally used among miners who speak the English language. In the gossan, silver ores are usually converted into chloride, bromide, etc., associated with various-shaped masses of native silver. Sulphide of copper is con verted into oxides, and then into malachite, azurite, the green and blue carbonates, and into chrysocolla, the green hydrous silicate. On account of the disintegration which has taken place the gossan is more economically worked than that part of the vein which lies below the water level, and is in its natural state, and therefore the first workings of min eral veins are generally the most profitable.