Fig. 15.—Ideal section of a vein.
Frequently, perhaps usually, the minerals of composite veins are deposited in succession, instead of cotemporaneously, giving rise to the remarkable banded structure so characteristic of this class of veins. The first mineral deposited in the fissure forms a layer covering each wall, and is in turn covered by layers of the second mineral, and that by the third, and so on, until the fissure is filled, or the solution exhausted. The distinguishing features of this structure are shown in [Fig. 15], in which w w represents the wall-rock, a a, b b, c c are successive layers of quartz, fluorite and barite, and the central band, d, is galenite. Since the vein grows from the outside inward, the outer layers are the oldest, and the central layers are the newest; again, the layers are symmetrically arranged, being repeated in the reverse order on opposite sides of the middle of the vein; and, lastly, in layers composed of prismatic crystals, as quartz (see the figure); the crystals are perpendicular to the wall and often project into, and even through, the succeeding layers. Such a crystalline layer is called a “comb” and the interlocking of the layers in this way is described as the comb-structure of the vein. The banding of veins is thus strongly contrasted with stratification, and with the structure in dikes due to the more rapid cooling along the walls. The duplicate layers are often discontinuous and of unequal thickness, on account of the strong tendency to segregation in the materials. This is clearly shown in [Fig. 16], drawn on a reduced scale from a polished section of a lead vein in Cumberland, England, contained in the Museum of the Boston Society of Natural History. In this the gangue minerals are fluorite (f) and barite (b). The central band (f g) is a darker fluorite containing irregular masses of galenite. The banded structure of veins is exactly reproduced in miniature in the banding of agates, geodes, and the amygdules formed in old lavas. Unfilled cavities frequently remain along the middle of the vein. When small, these are known as “pockets.” They are commonly lined with crystals; and when the latter are minute, the pockets are called druses. In metalliferous veins, the ore is much more abundant in some parts than in others, and these ore-bodies, especially when somewhat definite in outline, are known in their different forms and in different localities, as courses, slants, shoots, chimneys, and bonanzas of ore. The intersections and junctions of veins are often among the richest parts, as if the meeting of dissimilar solutions had determined the precipitation of the ore.
Fig. 16.—Section of a lead vein, one-fifth natural size.
Metalliferous veins, especially, are usually deeply decomposed along the outcrop by the action of atmospheric agencies. The ore is oxidized, and to a large extent removed by solution, leaving the quartz and other gangue minerals in a porous state, stained by oxides of iron, copper, and other metals, forming the gossan or blossom-rock of the vein.
Peculiar Types of Veins.—In calcareous or limestone formations, especially, the joint-cracks and bedding-cracks are often widened through the solution of the rock by infiltrating water, and thus become the channels of a more or less extensive subterranean drainage, by which they are more rapidly enlarged to a system of galleries and chambers, and, in some cases, large limestone caverns. The water dripping into the cavern from the overlying limestone is highly charged with carbonate of lime, which is largely deposited on the ceiling and floor of the cavern, forming stalactitic and stalagmitic deposits. These are masses of mineral matter deposited from solution in cavities in the earth’s crust, and are essentially vein-formations. Portions of caverns deserted by the flowing streams by which they were excavated, are often filled up in this way, being converted into irregular veins of calcite. But calcite is not the only mineral found in these cavern deposits, for barite and fluorite, and various lead and zinc ores, especially the sulphides of these metals—galenite and sphalerite—have also been leached out of the surrounding limestone and concentrated in the caverns. The celebrated lead mines of the Mississippi Valley, and some of the richest silver-lead mines of Utah and Nevada are of this character. The forms of these cavern-deposits vary almost indefinitely, and are often highly irregular. The principal types are known as gash-veins, flats and sheets ([Fig. 17]), chambers and pockets.
Where joints and other cracks have opened slightly in different directions and become filled with infiltrated ores, we have what the German miners call a stock-work,—an irregular network of small and interlacing veins.
Fig. 17.—Gash-veins and sheets.