The cavities in which the ores are precipitated have been formed in several different ways. The small rue.. when existing in sedimentary rocks, are probably due to the presence of pores between the grain-, or in the case of igneous rocks are gas cavities. Alure extensive ones may be caused by solution, especially in limestones. where caves are often formed by the solvent action of water, by contraction upon cooling or drying, which divides the rock along joint planes. or by folding or faulting of the strata, which may open fissures of considerable depth and length. The presence of cavities is not. however, necessary for the dep osition of ore, since the conditions which are sometimes favorable to its precipitation may also favor the solution of other minerals, and thus the particles of a common rock may be slowly dissolved while ore is precipitated in its place.
This process. which is known as replacement or metasomatism, is not uncommon, and often involves a large mass of rock. some deposits formed wholly in this manner. The boundary of a deposit formed chiefly in a cavity may sometimes he indefinite because replacement of the walls by ore has taken place.
Foam. Ore deposits vary greatly in their form, and this character has sometimes been used as the basis of classification. forms are so numerous as to deserve special mention.
Veins are filled fissures sometimes extend ing vertically, but more often inclined toward the horizontal. They have commonly been filled by deposition from solution, and in this respect differ from dikes (q.v.). In the true fis sure-vein the ore deposit, which is tubular in form, shows a banded structure due to the deposition of successive layers on the walls of the cavity. These layers or bands may represent different ores or consist of alternating layers of ore and gangue. At times, especially when the fissure is very nar row, the ore-bearing solutions may not only have filled the fissure. but have penetrated the wall rock as well. either filling the pores, or more often replacing portions of it, and then there will lie a gradation from vein matter to the wall rock. Where the fissure is extremely narrow the ore owes its presence almost entirely to replacement ; such veins lack a banded structure. being thereby distinguished from those formed by filling alone. If the vein is inclined the lower wall is spoken of as the foot-avail, and the upper one as the hanging wall. Veins often split, narrow (pinch), or widen (swell). and change their direction. in passing through hard, massive rocks like quartz ite, the vein fissure is apt to be clean-cut, but where the fissure passes through soft rock like shale or some dikes it may split up into a number of small stringers. A parallel series of closely
spaced veins is termed a Ionic. Veins may often intersect, and of two which cross each other one may be of a later date and follow a fault plane which has broken and displaced time earlier one. The ore in such cases is apt to be much richer at time point of intersection. Even in a single vein the ore may follow certain streaks which are termed shutes, or again it may be re stricted to pockets of great richness. which are known as bonanzas. Of the different gangue minerals found in veins, quartz is the commonest; in some regions, as California, quartz veins are extremely abundant and often carry considerable gold. Since the quartz is commonly resistant to the weather, the wail rocks may often decom pose and wash away, leaving the outcrop of the vein extending as a ledge or ridge along the sur face. Veins vary in width from a few inches to several hundred feet and their outcrop or apex A (q.v.) sometimes traceable for a long distance. Gash reins are a special type of local extent, formed by the enlargement of joint fissures.
The manner in1 which fissure veins have been filled, and the source of the metals which they contain, has been a subject of prolonged discus sion among economic geologists. Some have argued for the theory of lateral secretion, con sidering that the waters which carried the metals had dissolved them from the immediate walls of the fissure on either side of the vein. Others combat this view and claim that the solu tion of the theory of lateral secretion, the idea having been first brought forward in studying the deposits of Leadville, Colo.
The term imprcynatioa is applied to those de posits which have been formed by the deposition of ore in minute cavities of porous rock or in the crevices of a breecia. Or chamois are de posit-, formed along some path of easy access to the mineral solutions, as along the boundary be tween two kinds of rock. Ileddid deposits are formed parallel to the stratification of sedi mentary rocks. Contact deposits, as now defined, represent ore bodies formed along the contact of a mass of igneous and sedimentary rock, the ore having been derived wholly or in part from the intrusive masses. Chamber deposits include those formed in caves of solution. Placer dc posits include a series of gravel deposits of sedi mentary origin. These are widely distributed, especially in the Western States, and are a com mon source of gold, platinum, and sometimes tin.