Fig. 444.—Diagram showing effect of faulting on outcrops of anticlinal beds.
Fig. 445.—Diagram illustrating the effect of diminishing throw on outcrops in regions of folded rocks.
If a fault crosses folds at right angles to their axes, the effect is to change the distance between the outcrops of a given bed on opposite sides of the fault, after the truncation of the folded beds. The distance is decreased on the upthrow side of a syncline ([Fig. 443]) and increased on the upthrow side of an anticline ([Fig. 444]). If the throw of a fault in tilted beds diminishes in one direction, it may cause beds to outcrop, as shown in [Fig. 445]. Various other complications arise under other circumstances. Since faults rarely show themselves in the topography of the surface, except under special circumstances (see [p. 151]), their detection and measurement is usually based on the study of the relations of the beds involved, as illustrated by Figs. [436–445].
CHAPTER IX.
THE MOVEMENTS AND DEFORMATIONS OF THE EARTH’S BODY (DIASTROPHISM).
The body of the earth is subject to an infinite variety of movements, ranging from the almost inconceivably rapid to the almost imperceptibly slow, and from the almost immeasurably minute to the enormously massive; but, for practical treatment, they fall mainly into two couplets: (1) the minute and rapid, and (2) the slow and massive. Sudden movements of local masses, giving rise to intense vibrations, are put in the first class. There are innumerable minute and slow movements, but unless they rise to appreciable magnitude by long continuance, they are neglected.
MINUTE AND RAPID MOVEMENTS.
The crust of the earth is in a state of perpetual tremor. For the most part, these tremors are too minute to be sensible, but are revealed by delicate instrumental devices. Some of them are but the declining stages of sensible vibrations, but others are minute from their inception. Many of them spring from the ordinary incidents of the surface, and claim attention chiefly as obstacles to the study of more significant oscillations. Winds, waves, waterfalls, the tread of animals, the rumble of traffic, the blasts of mines, the changes of temperature, the variations in atmospheric pressure, the weighting of rainfall and the lightening of evaporation, the rupture of rock or ice or frozen earth, and many other processes, make their contributions to local and minute movements. For the greater part, these vibrations are superficial in origin, and are soon damped beyond recognition by dispersal and by the inelastic and discontinuous nature of the looser material of the surface. When a temporary rigid crust is formed by freezing, as in winter, these surface vibrations are transmitted with much less loss, and the distances at which the rumble of winter traffic is heard, is a good illustration of the function of continuity and solidity in the conveyance of vibrations.