Fig. 363.—Pre-Cambrian fossiliferous slate. Deep Creek Canyon, 16 miles southeast of Townsend, Mont. (Walcott, U. S. Geol. Surv.])
(1) Slaty structure.—When rocks made up of clastic particles are compressed in a given direction and are relatively free to expand at right angles to the direction of pressure, the particles that are already elongated tend to take positions with their longer axes at right angles to the direction of pressure, and all particles, whether elongate or not, are more or less flattened in a plane transverse to the direction of pressure. This may be readily seen where the particles are large ([Fig. 362]). As a result of the orientation and flattening of their particles, rocks so affected split more readily between the elongate and flattened particles than across them. In other words, the rocks cleave along planes normal to the direction of compression, and break with difficulty and with rough fracture across the planes of cleavage. The condition thus induced is known as slaty structure ([Fig. 363]), and is best illustrated by roofing-slate, which was originally a mud, later a shale, and finally assumed the slaty condition under strong compression. Sometimes the original bedding may still be seen running across the induced cleavage planes ([Fig. 364]). As the original mud beds were horizontal or nearly so, and as the thrust is usually horizontal or nearly so, the induced cleavage commonly crosses the bedding planes at a high angle ([Fig. 364]); but after the beds are tilted or bent, the lines of pressure take new directions relative to the bedding planes, and the angles between the original bedding and the slaty cleavages usually become smaller, and may even disappear in exceptional cases. Limestones, sandstones, and conglomerates are not so easily compressed as mudstones, and they usually take on only an imperfect cleavage normal to the direction of pressure. Often they merely show some little compacting, while the shaly strata between them are converted into slate. Obviously the direction of slaty cleavage may be used to determine the direction of the compressing force, and is thus serviceable in dynamic studies.
Fig. 364.—Slaty structure and its relation to bedding planes. Two miles south of Walland, Tenn. (Keith, U. S. Geol. Surv.)
Fig. 365.—Foliated rock. (Ells, Can. Geol. Surv.)
Foliation, schistosity.—A more intense application of pressure in a given direction is capable of breaking down and deforming the most resistant rock. This must necessarily be attended with the evolution of much heat, and thermal effects are mingled with pressure effects, but the thermal effects may be neglected for the moment. The first stage of the mechanical effect of the compression may be to crush the rock more or less. It thus becomes granular or fragmental, and is really a peculiar species of clastic rock (autoclastic). At a further stage, the fragmented material may be pressed into layers or leaves, much as in the development of slaty cleavage, but as a result of the nature of the material, the cleavage is less perfect. This is often attended by more or less shearing of the material upon itself, and thus a rude fissility and foliation is developed. The result, including the attendant metamorphism about to be described, is a foliated or schistose structure (Figs. [365] and [366]). Even the most massive rocks may be reduced to the foliated form by this process; thus, a granite may be mashed into a gneiss—which is a granite in composition, but has a foliated structure—or a basalt may be converted into a schist, a common term for foliated crystalline rocks. Porphyritic rock rendered schistose by pressure is shown in [Fig. 366]. When massive rocks like granite or basalt are thus crushed down into the foliated form, the process is in a sense degradational. It is a kind of katamorphism or downward change. It is often difficult to differentiate the schists thus derived by degrading massive rocks, from those developed by ascensional processes from clastic formations (anamorphism). The action of heat is important in the evolution of schists of both classes, but the effects of heat may best be taken up where it acts measurably alone.
Fig. 366.—Porphyry rendered schistose by pressure. Near Green Park, Caldwell Co., N. C. (Keith, U. S. Geol. Surv.)