Fig. 410.—Plicated layers of thin-bedded chert in limestone, etched by erosion. Lower Cambrian (?), two miles southwest of Big Pine, Inyo Co., Cal. (Walcott, U. S. Geol. Surv.)
Joints.—The surface rocks of the earth are almost universally traversed by deep cracks called joints (Figs. [415], [138] and [140]). In most regions there are at least two systems of joints, the crevices of each system being roughly parallel to one another, while those of the two systems, where there are two, are approximately at right angles. In regions of great disturbance, the number of sets of joints is often three, four, or even more. The joints of each set may be many yards apart, or in exceptional cases, but a few inches, or even a fraction of an inch.
Generally speaking, there are more systems of joints, and more frequent joints in each system, where the rocks are much deformed than where they have been but little disturbed. In undisturbed rocks the joints approach verticality, but in regions where the rocks have been notably deformed, the joint planes may have any position. Not rarely they simulate bedding planes, especially in igneous and metamorphic rocks ([Fig. 416]). In the latter case especially, the cleavage due to jointing is often mistaken for bedding. They do not ordinarily show themselves at the surface in regions where there is much mantle rock, but they are readily seen in the faces of cliffs, in quarries, and, in general, wherever rock is exposed (Figs. [138] and [140]). Though some of them extend to greater depths than rock has ever been penetrated, joints are, after all, superficial phenomena. They must be limited to the zone of fracture, and most of them are probably much more narrowly limited. Joints frequently end at the plane of contact of two sorts of rock. Thus a joint extending down through limestone may end where shale is reached. Joints are frequently offset at the contact of layers or formations, and a single joint sometimes gives place to many smaller ones. All these phenomena are to be explained on the basis of the different constitution and elasticity of various sorts of rock. Generally speaking, rigid rock is more readily jointed than that which is more yielding.
Joints may remain closed, or they may gap. In the latter case, they may be widened by solution, weathering, etc., but they are quite as likely to be filled by detritus from above, or by material deposited from solution (veins). It is along joint-planes that many rich ore-veins are developed ([pp. 478–484]).
Fig. 411.—This diagram might represent either isoclinal or monoclinal structure. In the former case the strata might have the structure shown in any one of the following Figures, 412 to 414, so far as dip and strike show. (Dana.)
Fig. 412.
