Common Minerals and Rocks

by William O. Crosby

Fig. 27.—Slaty cleavage in contorted strata.

The principal characteristics of lithologic cleavage are: (1) It is rare, except in fine-grained, soft rocks, having its best development in the slates, roofing slates and school slates affording typical examples. Hence it is commonly known as slaty cleavage. (2) The cleavage planes are highly inclined or vertical, very constant in dip and strike, and quite independent of stratification. (3) It is usually associated with folded strata, and often with distorted nodules or fossils. The more important of these characteristics are illustrated by [Fig. 27]. This represents a block of contorted strata in which the dark layers are slate with very perfect cleavage parallel to the left-hand shaded side of the block; while the white layers are sandstone and quite destitute of cleavage. Many explanations of this interesting structure have been proposed, but that first advanced by Sharpe may be regarded as fully established. He said that slaty cleavage is always due to powerful pressure at right angles to the planes of cleavage. All the characteristics of cleavage noted above are in harmony with this theory. Cleavage is limited to fine-grained or soft rocks, because these alone can be modified internally by pressure, without rupture. Harder and more rigid rocks may be bent or broken, but they appear insusceptible of minute wrinkling or other change of structure affecting every particle of the mass. Since the cleavage planes are normally vertical, the pressure, according to the theory, must be horizontal. That this horizontal pressure exists and is adequate in direction and amount, is proved by the folds and contortions of the cleaved strata; for, as shown in [Fig. 27], the cleavage planes coincide with the strike of the foldings, and are thus perpendicular to the pressure horizontally as well as vertically. The distortion of the fossils in cleaved slates is plainly due to pressure at right angles to the cleavage, for they are compressed or shortened in that direction, and extended or flattened out in the planes of cleavage. Again, Tyndall has shown that the magnetism of cleaved slate proves that it has been powerfully compressed perpendicularly to the cleavage. And, finally, repeated experiments by Sorby and others have proved that a very perfect cleavage may be developed in clay (unconsolidated slate) by compression, the planes of cleavage being at right angles to the line of pressure. When, however, Sharpe’s theory had been thus fully demonstrated, the question as to how pressure produces cleavage still remained unanswered. Sorby held that clay contains foreign particles with unequal axes, such as mica-scales, etc., and that these are turned by the pressure so as to lie in parallel planes perpendicular to its line of action, thus producing easy splitting or cleavage in those planes. And he proved by experiments that a mixture of clay and mica-scales does behave in this way. But Tyndall showed that the cleavage is more perfect just in proportion as the clay is free from foreign particles, and in such a perfectly homogeneous substance as beeswax, he developed a more perfect cleavage than is possible in clay. His theory, which is now universally accepted, is, that the clay itself is composed of grains which are flattened by pressure, the granular structure with irregular fracture in all directions, changing to a scaly structure with very easy and plane fracture or splitting in one definite direction.

Observations on distorted fossils and nodules have shown that when slaty cleavage is developed, the rock is, on the average, reduced in the direction of the pressure to two-fifths of its original extent, and correspondingly extended in the vertical direction. Thus, whether rocks yield to the horizontal pressure in the earth’s crust, by folding and corrugation, or by the flattening of their constituent particles, they are alike shortened horizontally and extended vertically; and it is impossible to overestimate the importance of these facts in the formation of mountains.

Faults or Displacements.—We may readily conceive that the forces which were adequate to elevate, corrugate, and even crush vast masses of solid rock were also sufficient to crack and break them; and since the fractures indicate that the strains have been applied unequally, it will be seen that unequal movements of the parts must often result. If this unequal movement takes place, i.e., if the rocks on opposite sides of a fracture of the earth’s crust do not move together, but slip over each other, a fault is produced. The two sides may move in opposite directions, or in the same direction but unequally, or one side may remain stationary while the other moves up or down. It is simply essential that the movement should be unequal in direction, or amount, or both; that there should be an actual slip, so that strata that were once continuous no longer correspond in position, but lie at different levels on opposite sides of the fracture. The vertical difference in movement is known as the throw, slip, or displacement of the fault. Fault-fractures rarely approach the horizontal direction, but are usually highly inclined or approximately vertical. When the fault is inclined, as in [Fig. 28], the actual slipping in the plane of the fault exceeds the vertical throw, for the movement is then partly horizontal, the beds being pulled apart endwise. The inclination of faults, as of veins and dikes, should be measured from the vertical and called the hade. Faults are sometimes hundreds of miles in length; and the throw may vary from a fraction of an inch to thousands of feet.

Fig. 28.—Section of a normal fault.

Fig. 29.—Section of a reversed fault.

Transverse sections, such as are represented by [Fig. 28] and many specimens and models, do not give the complete plan or idea of a fault; but this is seen more perfectly in [Fig. 30]. We learn from this that a typical fault is a fracture along which the strata have sagged or settled down unequally. The most important point to be observed here is that the strata do not drop bodily, but are merely bent, the throw being greatest at the middle of the fault and gradually diminishing toward the ends. In other words, every simple fault must die out gradually; for we cannot conceive of a fault as ending abruptly, except where it turns upon itself so as to completely enclose a block of the strata, which may drop down bodily; but the fault is then really endless. A fault may be represented on a map by a line; if a simple fault, by a single straight line. But faults are often compound, and are represented by branching lines; that is, the earth’s crust has been broken irregularly, and the parts adjoining the fracture have sagged or risen unequally.