Concretions also form in water during the deposition of sedimentary rock. Exceptionally, sedimentary rock is made up chiefly of concretions. The chemical precipitates from the concentrated waters of certain enclosed lakes sometimes take the form of minute spherules which resemble the roe of fish. From this resemblance the resulting rock is called oolite ([Fig. 357]). Oolite is now forming about some coral reefs, presumably from the precipitation of the lime carbonate which was temporarily in solution. Considerable beds of limestone are sometimes oolitic. The calcium carbonate of such rock may be subsequently replaced by silica, so that the oolitic structure is sometimes found in silicious rock. If the concretions become larger, say as large as peas, the rock is called pisolite instead of oolite ([Fig. 378]).
Fig. 378.—Pisolite. Half natural size. (Photo. by Church.)
Fig. 379.—Columnar structure, “Devil’s Post Pile.” Upper San Joaquin Canyon, Sierra Nevada Mountains.
Beds of iron ore are likewise sometimes concretionary. Thus in the Clinton formation there are widespread beds of “flaxseed” ore made up of concretions of iron oxide which, individually, resemble the seed which has given the ore its name. The nucleus in this case is usually too small for identification.
Secretions.—When cavities in rock are filled by material deposited from solution, the result is sometimes called a secretion. Secretions therefore grow from without toward a center, while concretions follow the opposite order. Crystal-lined cavities (geodes, [Fig. 359]) and agates ([Fig. 358]) are examples of secretions. Crystal-lined cavities and veins are the same in principle.
STRUCTURAL FEATURES OF IGNEOUS ROCKS.
Certain structural features of igneous rocks have been mentioned in treating of their origin in the previous chapter. When a great flow of lava spreads out upon the surface, there is no internal lamination or stratification, and the resulting rock is usually classified as massive rather than stratified; but when a succession of flows occur, each individual flow forms a layer, and the series as a whole becomes stratiform. The successive flows are not usually coextensive. If the later flows of the closing stages of a period of vulcanism fail to reach as far as the earlier ones, a terraced or step-like aspect is given to the region, whence the name trap-rock (trappe, steps) is derived. Such lava sheets, especially if of basalt, often assume a columnar structure in cooling, the columns being rude six-sided prisms standing at right angles to the cooling surfaces (Figs. [379] and [380]). This phenomenon is usually best developed where the sheet is intruded between layers of preexisting rock in the form of sills. The formation of the columns is sometimes regarded as a variety of concretionary action, but more commonly as a result of contraction. The former is suggested by the ball-and-socket ends of the sections of some columns ([Fig. 382]). The development of the columns by contraction may be explained as follows: The surface of the homogeneous lava contracts about equally in all directions on cooling. The contractile force may be thought of as centering about equidistant points. About a given point, the least number of cracks which will relieve the tension in all directions is three ([Fig. 383]). If these radiate symmetrically from the point, the angle between any two is 120°, the angle of the hexagonal prism. Similar radiating cracks from other centers complete the columns ([Fig. 384]). A five-sided column would arise from the failure of the cracks to develop about some one of the points ([Fig. 385]).
