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There is a distinct relationship between the length and depth of veins; in general, long, strong, wide veins persist in depth and short, non-continuous, irregular and weak veins die out at no great distance beneath the surface, perhaps to be followed by similar and roughly parallel veins in depth. While ore-bearing fissures are commonly not important as faults, there is a relation between the amount of throw and the length and depth of a fissure; the greater the throw, the greater are likely to be these dimensions.

Fractures produced by compressive stress through homogeneous rocks are likely to form systems parallel in strike but opposite in dip, known as conjugate fractures; where mineralized these become conjugate veins, of which one dip is usually emphasized while the other is subordinate, or exists as a simple fissure only. Conjugate joint systems are of common occurrence. The intersection of two systems of intense jointing appear to be favorable loci for ore deposition.

Certain deposits take the form of a pipe or chimney, and have marked vertical continuity, with very subordinate horizontal dimensions. It is difficult to conceive of a fissure whose only important dimension is approximately vertical, and these deposits were probably formed at the intersection of fissures that throughout the remainder of their lengths did not permit the passage of mineralizing solutions, and so failed to be emphasized in connection with the main deposit; other deposits of this form appear to be due to the mineralization of fumarolic vents.

A zone of faulting in which mineralization has taken place irregularly through the crushed and comminuted fault material is known as a fault lode. This type of deposit, where the values are irregularly distributed either with or without a scanty gangue, is not uncommon in the areas of old schists in the desert region of the Southwest, These lodes are difficult to follow, and their exploration in advance of actual mining is usually unprofitable.

A sheeted zone is a series of closely set parallel sheeting planes; they frequently afford channels for mineralizing solutions and so become mineral lodes. Such lodes are commonly not as persistent as fissure veins, but mineral deposits may follow them for long distances, the mineralization being found sometimes in one and sometimes in another set of sheeting planes, following, perhaps, an overlapping arrangement. Sheeting planes are commonly parallel to the main fissures, and lodes are frequently made up of a mineralized fissure with one or more mineralized sheeting planes nearby.

A bed vein is a vein that follows a bedding plane of an enclosing sedimentarj'- rock, less frequently a plane between layers of volcanic rocks. Bed veins are commonly thought to be less persistent than veins that cut across the strata of enclosing rocks; many cases are known, however, where bed veins are both persistent and contain important ore-shoots. Blanket vein is often used as a sjTionym for bed vein, but actually refers to a horizontal or nearly horizontal position only.

A lode is a zone of fissuring that contains roughly parallel mineral masses of the general type of fissure veins, usually connected by cross veins and mineralized breccias to such a degree that over certain portions the whole width constitutes a single ore-body.

A ledge is an irregular mass of altered rock, containing ore bodies, the alteration of which is due to and characteristic of the action of mineralizing solutions.

These veins are divided by Mr. Lindgren into four classes: (a) Contact deposits, which are discussed elsewhere.
(b) Cassiterite veins. The characteristic minerals of this type are cassiterite, pyrite, arsenopyrite, specularite, quartz, tourmaline, topaz, lepidolite, muscovite, apatite, fluorite, and wolframite, with subordinate calcite and siderite. These veins are commonly poor in gold and silver, and the metasomatic alteration along their walls is likely to be intense.

Primary mineral deposits formed at the surface by hot waters are rarely of economic importance; their primary condition is commonly obscured by the action of surface agencies. The sinters characteristic of surface-formed deposits are commonly made up of silica, as opal or chalcedony, and earthy carbonates. Calcite, fiuorite, celestite, barite, and many other gangue minerals may also develop in crystallized form. Stibnite, pyrite, marcasite and cinnabar are known in crystallized form, and many other sulphides have been detected chemically in such deposits.

The only primary ore of zinc of importance is zincblende; in characteristic occurrences it is associated with pyrite, and occasionally with galena and chalcopyrite. The Depth of Primary Ore Deposition.—An important factor in the consideration of the probable behavior of a primary deposit in advance of exploration is the depth at which it formed.