Fig. 357.—Oolitic texture. About natural size. (Photo. by Church.)
Fig. 358.—Agate structure. The cavity was first coated with mineral matter deposited from solution. The contracted cavity was then nearly filled with the same sort of material deposited in layers, apparently over the bottom, until the cavity was nearly obliterated.
Cavity filling.—When cavities of some size occur in rocks and the percolating waters are in a depositing state, the interiors of the cavities are sometimes lined with concentric layers of deposit. Here, instead of building out from a nucleus, the waters build in from the walls of the cavity. The agate structure ([Fig. 358]) is a case of this kind, in which the successive layers are commonly silica in the form of chalcedony and differ from each other in color and texture. Often before the cavity is entirely filled, the deposit changes from chalcedony, to crystals of quartz, which grow with their bases on the walls and their pyramidal points toward the center of the cavity. Geodes are examples of a similar process in which the cavity is but partially filled with crystals which have their bases set on the walls of the cavity and their points directed inwards ([Fig. 359]). The crystals of geodes are most commonly quartz or calcite, but they may be any other mineral that the waters are capable of depositing. Very large cavities lined in this way are known to miners as vuggs, and these grade on into caves lined with crystals and with stalactite and stalagmite. These are the largest expression of the solidifying process by means of internal deposition.
Fissure-filling; veins.—Cracks, crevices, and fissures filled by deposition in a similar way give rise to veins ([Fig. 360]). Here the filling grows from the walls toward the center, and hence often has a banded appearance. By this filling of cracks and crevices, the circulating water heals the breaks in the rocks. Frequently a crushed zone is thus restored to a solid state. When the fissures are deep and wide and traverse different formations, conditions are afforded for very complex deposits, and for the concentration of rare and valuable material originally dispersed through a great mass of rock. Ore deposition in such veins is usually treated as a theme by itself, but it is really but a declared expression of the work which the percolating waters are doing throughout all the rocks which they penetrate. Most of the fine crystals that grace mineralogical collections were formed in cavities and fissures by deposition from circulating mineralized waters.
Fig. 359.—A geode. About half natural size. (Photo. by Church.)
Solution as well as deposition.—A further phase of the process needs attention. The percolating waters are constantly taking up matter as well as throwing it down, and so, while they are cementing fragments together and healing fractures, they are also removing material, and a rock may be growing porous and cavernous at the same time that its fragments are being united. Cavities may be formed at one stage and filled at another; matter may be taken up at one point and put down at another, and so an internal reconstruction is in slow progress.
Fig. 360.—Veins of calcite in limestone. Calciferous formation near Highgate Springs, Vt. (Walcott, U. S. Geol. Surv.)