CHAPTER VIII.

FAULTS OR DISLOCATIONS.

IF 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 produce 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.

Fig. 93. Faults with the same amount of throw.

Throw and Hade of Faults. Suppose fig. 93 to be a vertical section through a mass of rock dislocated by two faults, ss 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 f 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. 93, the

mass resting upon the base, b a, may be supposed to have been bodily upraised.

The amount of displacement produced by a fault is called its throw, and is always measured by the vertical distance between the broken ends of any given bed. Where the plane of the fault is vertical, as fa, in fig. 93, 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 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

α

Fig. 94.

of the beds displaced is not always a measure of the throw of a fault; when the beds are inclined, as in fig. 94, the 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 thickness of the beds displaced. Whether it is more or less than this thickness will depend on the angle which the plane of the fault makes with the plane of the dipping stratum.

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 to the horizon, i.e., from f to b, in figs. 93 and 94, is called its hade or underlie. Thus we speak of the dip of a bed and the hade of a fault. Faults usually hade at a high angle, from 45° to 60° being common inclinations; they are sometimes vertical, as at fa in fig. 93, and occasionally the angle of hade is so low as 20o.

There is also a relation between the hade of a fault and the direction of its throw; as a general rule a fault hades under the downthrow side, so that the ends of the severed beds are separated by a greater or less horizontal distance. It also follows under this rule that no part of a severed bed is brought vertically underneath another part of it.

C

Fig. 95. A reversed Fault.

There are, however, exceptions to this rule, and when a fault hades under the upthrow, as in fig. 95, bringing one portion of the bed, e, below the other portion, it is called a reversed fault. Faults of this kind are not uncommon where anticlinal folds have been broken and dislocated, and in such cases it would appear that the whole mass of beds on the upthrow side has been pushed up the inclined plane of the fault.

Effects of Faults on the Rocks which they traverse. Small faults may sometimes be observed where the beds have been broken through so cleanly and evenly that they look as if they had been cut with a knife, the opposing walls of the fissure fitting closely and regularly to one another. More generally, however, the contiguous strata have been bent, crumpled, and pulled down along the plane of the fault to a certain extent, as in fig. 96. The

beds are here represented as bent upon the downthrow side, or in mining language as "rising towards the upthrow," and "dipping towards the downthrow," and this is naturally the most usual effect of a forcible dislocation, accompanied or followed by lateral pressure; but it is not invariably the case, instances occurring where the beds have

Fig. 96. Flexure of Beds near a Fault.

been bent in an opposite direction, so as to curve upwards on the upthrow side.

Mr. Jukes has described a case of the latter kind as seen in the gate-road of a colliery at Himley in South Staffordshire. The beds here brought into contact by this fault were the Coal-measures and certain red rocks (known as Permian). Mr. Jukes says, "Both formations were greatly

Fig. 97. Section through a Fault at Himley (after Jukes). A, The Thick coal squeezed and bent upwards; B, Ironstones and shales; c, The Heathen coal; D, Permian beds.

shattered and broken, so much so that no trustworthy determination of the dip of the Permian beds could be made. The coal-measures were not only shattered but squeezed, so that the thick coal lost much of its usual thickness on approaching the fault, and when it came within 10 or

12 yards of it, it was bent up perpendicularly, and the beds below it rose into the walls of the gate-road, and were cut off above by the red rock lying obliquely across them at an irregular line, the inclination of which to the horizon did not exceed 37°, somewhat as in the preceding figure [fig. 97], which is condensed from a rough sketch and measurement I made on the spot."

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.

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