Some gold areas may show parallel planes of fracture, developed by compressive stresses, of relatively great continuity as compared with joints, but of only incipient displacement, are called sheeting planes. Closely set and well-developed sheeting planes often afford channels for the circulation of solutions, and, when mineralized, form sheeted lodes. Not infrequently systems of sheeting planes occur in pairs, parallel in strike, but intersecting in dip, such fracturing being the typical result of compressive or torsional strains; these interdependent systems of sheeting planes are known as conjugate systems.
It is important to remark that fold is a bend in a rock mass caused by compressive stress of insufficient intensity to produce a fault. A fold is called a syncline if its bend is concave above and an anticline if its bend is concave below. A dome is an anticline whose length is zero and is best described by the usual meaning of its name. A basin is the synclinal equivalent of a dome. Folds are commonly persistent in strike, and domes or basins are of relatively rare occurrence. A monocline may be described as a half-fold, as where strata assume a terrace-like position, being parallel at different elevations either side of an inclined connecting part. The rock forming the convex or outer portion of a fold is, like the lower portion of a beam subjected to load, under tensile stress, which frequently results in a series of fractures that may afford channels for the circulation of solutions and become the place of gold deposition.
Where a fold is formed by compressive stress beyond the capacity of the rock to withstand, a fault is developed along the axis, or the plane bisecting the angle between the component limbs of the fold. Folds' not infrequently pass into faults along their strike, and in a region that has been subjected to both folding and faulting these expressions of stress are likely to be parallel. The term fracture denotes a break in a rock mass in importance intermediate between a joint and a fault, as these latter terms are generally used by gold prospectors. Also, a flexure is a sharp bend in a series of strata or in a rock mass, without the development of a continuous fracture, the result being a displacement similar to that of a monocline. Flexures readily pass into faults, which are fractures through a rock mass the opposite walls of which have moved past each other, the word indicating lack of correspondence between opposite walls.
The strike of a fault is the direction of a horizontal line within the plane of the fault; the dip of a fault is the angle between the plane of the fault and a horizontal plane. The distance measured on the plane of a fault between the new positions of two points that were originally opposite is called the total displacement, which distance may, for purposes of calculation, is considered as the hypotenuse of a right triangle, whose sides represent the horizontal and the vertical movements of which it is the resultant. Also, the terms throw and heave as commonly used refer respectively to the vertical and horizontal distances between the new positions of originally opposite points in a fault the movement along which was directly down the dip; by offset is commonly meant the horizontal distance (perpendicular to the strike) between the two portions of a faulted vein, without regard to the direction or amount of total displacement. In a great majority of faults the total displacement is not directly down the dip, but is the result of both horizontal and vertical movements.
It has been established that a normal fault is a fault along which the hanging wall has moved downward on the foot-wall, and is the natural result of a drawing a part of the two rock masses, the least supported mass slipping downward on the mass having the larger base. Normal faults are commonly the effect of gravity and due to tension although they sometimes result from compressive stresses. Faults due to tension commonly find expression in a simple fracture and are not accompanied by sheeting.