DISLOCATION, or FAULT, a term used in geology to characterize certain displace ments common among stratified rocks. The agency that raised these rocks above the waters of the sea. produced in the elevation numerous rents. In their simpleq form, the rents are mere cracks, the parts, though separated, remaining contiguous; sometimes a greater or less fissure intervenes between the disunited portions, and this is filled with materials pressed in from above, or with igneous rocks intruded from below. The beds, however, are not always found at the same level—a displacement as well as a severance often takes place, so that the beds on one side of the fissure or crack are many feet, or many hundreds of feet, above or below the beds on the other side with which they were once continuous. One of the best known faults is that called the "Ninety Fathom Dike" in the Newcastle coalfield. The same beds are 90 fathoms lower on the northern than they are on the southern side. The fissure varies in width from a few feet in some places to more than 20 yards in others; it is filled with sandstone. In the Edinburgh coalfield, the greatest fault is the one known as the "Sheriffhall Slip." It has pro duced a dislocation of the strata to the extent of 400 or 500 ft., so that the coal is worked on the s. side of the slip, near the surface, is on the n. side 500 ft. below it. Mr. Milne Home enumerates 120 faults in the Mid-Lothian and East-Lothian coal-fields.
He has himself examined 78 of these, and has found that 35 dip to the s., and 43 to the n.; and that while the sum of the down-throws by the faults dipping to the s. is 385 fathoms, those dipping to the n. have depressed the strata 754 fathoms.
Faults in coal-fields are well known, because of their serious interference with the progress of the miner. But though they often cause considerable labor and expense in searching for the continuation of a valuable seam of coal, they have corresponding advantages, since they disclose on the surface the value of the buried minerals, and when filled with solid materials, they form embankments which confine -water, and thus save considerable expense in draining the mine.
The amount of dislocation is the measure of a line drawn from one part of the bed, at right angles to its plane, to a line produced from the other separated part of the bed representing its plane. Although no notion can be formed, in meeting with a fault, of the extent of dislocation, yet the direction in which the lost strata are to be sought can be certainly determined, for it has been found to be an invariable law, that the strata are lowest on the overlapping side of the slip. Faults have local names from the miners, all of which have been used by geologists. They are called hitches, dikes, troubles, slips, slides, heaves, and throws.