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CHAPTER IX.

FAULTS OR DISLOCATIONS.

1. Direct or Normal Faults.

F great thickness of hard and solid rocks can be bent into the curves and contortions described in the last chapter, it may easily be conceived that a different application of the same force would be capable of cracking and breaking through them. We find accordingly that rocks are often traversed by great cracks or fissures, which pro

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Fig. 123. Faults with the same amount of Throw.

duce not only a severance, but a displacement of the beds on either side, so that the separated portions are raised or depressed far above or below those with which they were originally connected. These fractures and displacements of large masses of rock are called dislocations or faults.

Throw and Hade of Faults.-Suppose fig. 123 to be a vertical section through a mass of rock dislocated by two faults, 8 8 being the surface of the ground, and c c the course of a single bed which was once continuous, but has

been broken in two places by the two faults ƒ a and ƒ b. The two sides of any fault are spoken of as the up-cast and down-cast sides, though it is generally impossible to say whether the dislocation has resulted from the depression of one side or the upheaval of the other. In fig. 123, the mass resting upon the base, b a, may be supposed to have been bodily upraised.

To indicate the amount of displacement caused by a fault miners speak of its throw, and always measure this by the vertical distance between the broken ends of any given bed. Where the plane of the fault is vertical, as ƒ a, in fig. 123, the amount of throw is easily measured along it; but when the plane of the fault is inclined to the horizon, as at ƒ b, the throw is measured by prolonging the level of the given bed till a vertical line will reach its continuation either

f

Fig. 124.

above or below it. In the case figured the throw of each fault is supposed to be the same, viz., 100 feet. The beds are also drawn horizontal, and when this is the case the amount of throw is also measured by the thickness of the beds displaced by the fault, i.e., the thickness between b and d.

The student must observe, however, that the thickness of the beds displaced is not always a measure of the throw of a fault; when the beds are inclined, as in fig. 124, the vertical throw of the fault, measured from the level of one end of the broken bed to the level of its continuation, i.e., from a to b, is evidently less than the the thickness of the beds displaced.

Miners also in describing the "width of a fault" mean the horizontal distance between the broken ends of a bed, namely, the distance c d in fig. 123. Geologists require a

more logical nomenclature, and often want to express the amount of stratigraphical displacement, as measured by the thickness of beds displaced, Messrs. Margerie and Heim' have therefore proposed to distinguish the three different measures of displacement above mentioned by the terms vertical throw, horizontal throw, and stratigraphical throw. Thus, in fig. 124 the vertical and horizontal throw are nearly equal, but the stratigraphical throw is much greater than either.

The amount of displacement or "throw" varies from a few feet to many hundred feet; faults involving a displacement of even several thousand feet having been found in some places, so that rocks of widely different kinds and ages are brought into apposition with one another. Thus, there is every gradation between cracks and fissures, which are merely enlarged joints with hardly any vertical displacement, and dislocations on the great scale above mentioned.

The inclination of the plane of the fault from the vertical, i.e., the slope from ƒ to b, in figs. 123 and 124, is called its hade or underlie. Thus we speak of the dip of a bed and the hade of a fault, but their angles are not measured in the same way. Faults usually hade at a high angle, only 20° or 30° from the vertical being common inclinations; and they are sometimes vertical, as at ƒ a in fig. 123.

Open and Close Faults.-When faults traverse soft and yielding beds of rock, such as shales and thin sandstones or limestones, the fissures themselves are often mere planes of division no thicker than a knife-blade.

When faults traverse very hard and unyielding rocks, such as thick limestones or hard flagstones, and still more if they penetrate hard igneous or metamorphic rocks, the fissures are apt to be much wider and often very irregular. If the original fracture has taken place, not in one plane, but so as to produce two uneven and irregular surfaces, these surfaces, in sliding one over the other, are not likely to fit very closely, but will leave hollows and spaces here and there between the two walls of the fissure.

1 "Dislocations de l'écorce terrestre," Zurich, 1888, p. 17.

It is true that the grinding process, as the rock-surfaces moved across each other, would often greatly diminish this irregularity, and, in soft rocks, probably obliterate it; but in hard rocks it is usual to find more or less space between the walls of the fault.

In all faults the contiguous surfaces are generally found to be polished and striated by the enormous friction which took place during the movement of one face across the other. These appearances are known by the name of slickensides, and it often happens that such slickensides occur on the surfaces of all the joints and cracks for some distance on either side of the fault, and are indicative of the jarring nature of the movement.

It must not be supposed, however, that such fissures are now open and empty. Sometimes they have remained open for a long time, and have been gradually filled up with deposits of crystalline mineral matter, in which case they are converted into lodes or mineral veins; these will be described on a future page. In other cases the spaces between the rock-walls are filled with a confused mass of fragments broken off the contiguous rocks during the movement of dislocation, mingled with sand and clay, produced by the attrition of these fragments against one another and the walls of the fault. This mass of fragmentary material is termed fault-rock, and is often consolidated into a kind of breccia.

Very large lumps and blocks of the broken beds are sometimes included in this way between the walls of a fault and serve to indicate the existence and direction of the fracture when it is being traced along the surface of the ground.

Single and Branching Faults.-In cases where the dislocation is affected by a single line of fault, it is clear that the beds must have been bent either upwards or downwards on one side of the fault, or upwards on one side and downwards on the other for a certain distance. Thus in fig. 125 some beds are supposed to have been cracked by the fissure a b, and the part c to have been bent down, but we might just as easily have supposed the part d bent up, or both operations to have taken place simultaneously. Without some such bending no dislocation could have occurred.

The existence of such single-line faults is often proved in coal-mining. Not unfrequently they split towards one or both extremities as in the plan, fig. 126, where the main fault is split into three branches at one end, and two at the other. The figures represent the amount of the downthrow at each point in yards. Such faults generally have one, but sometimes several points of maximum "throw

near the centre, and gradually diminish each way till they die out. To produce more than one point of maximum throw, we must suppose that there is an undulation of the beds along one or both sides of the fault.

Single lines of fracture are probably much more extensive than the spaces where they are made visible by the dislocation of the beds, since the bending which causes

Fig. 126. Plan of Fault splitting at the Ends.

dislocation is more likely to occur near the central part of a fracture than near its extremities. Thus in fig. 134 the fracture indicated by the line & is probably continued beyond the point where the line ends, though the actual dislocation ceases there.

When there is more than one line of fracture the fact of dislocation becomes more easy to understand, since there is no difficulty in conceiving that the angle or corner of ground included between the intersection of two faults, has