Studied throughout a considerable district, the various series which make up these two sets of master joints may be seen locally developed in different combinations as well as in association with additional fissure planes which are not easily reduced to any simple law of arrangement ([Fig. 38 a, a, a]). Only rarely are regular joint series observed which do not stand perpendicular to the original attitude of the rock beds. In a few localities, however, rectangular joint sets have been discovered which divide the rock into prisms parallel to the earth’s surface and with the joint series inclined to it each by half a right angle. Where the rock beds have been much disturbed, the complex of joints may be such as to defy all attempts at orderly arrangement.
Fig. 39.—View on the shore at Holstensborg, West Greenland, to show the subequal spacing of the joints (after Kornerup).
Fig. 40.—View of an exposed hillside in Iceland upon which the snow collected in crannies along the joints brings out to advantage both the larger and the smaller intervals of the joint system (after Thoroddsen).
The space intervals of joints.—The same kind of subequal spacing which characterizes the fractures near the surface of the block in Daubrée’s experiment ([Fig. 19], [p. 41]) is found simulated by the rock joints ([Fig. 39]). Such unit intervals between fractures may be grouped together into larger units which are separated by fractures of unusual perfection. We may think of such larger space units as having the smaller ones superimposed upon them ([Fig. 40]).
The displacements upon joints—faults.—In the vast majority of cases, the joint fractures when carefully examined betray no evidence of any appreciable movement of the two walls upon each other. Generally the rock layers are seen to cross the joints without apparent displacement. Joints are therefore planes of disjunction only, and not planes of displacement.
Fig. 41.—Faulted blocks of basalt divided by joints near Woodbury, Connecticut. To show the structure of the rock, some of the foliage has been removed in preparing the sketch from a photograph.
Within many districts, however, a displacement may be seen to have occurred upon certain of the joint planes, and these are then described as faults. Such displacements of necessity imply a differential movement of sections or blocks of the earth’s crust, the so-called orographic blocks, which are bounded by the joint planes and play individual rôles in the movement. A simple case of such displacements in rocks intersected by a single set of master joints is represented in the model of plate 4 C. The most prominent fault represented by this model runs lengthwise through the middle, and the displacement which is measured upon it not only varies between wide limits, but is marked by abrupt changes at the margins of the larger blocks. This vertical displacement upon the fault is called its throw. Though not illustrated by the model, horizontal displacements may likewise occur, and these will be more fully discussed when the subject of earthquakes is considered in the following chapter. An actual example of blocks displaced by vertical adjustment is represented in [Fig. 41], a simple type of faulting which has taken place in rocks but slightly disturbed from their original attitude, but intersected by a relatively simple system of master joints. In those regions where the beds have been folded and perhaps overthrust before their elevation into the zone of fracture, and which are further intersected by disorderly fissure planes, the results are far more complex. In such cases the planes of individual displacement may not be vertical, though they are generally steeper than 45°. For their description it is necessary to make use of additional technical terms ([Fig. 42]). The inclination of a sloping fault plane measured against the vertical is called the hade of the fault. The total displacement is measured along the plane of the fault from a point upon one limb to the point from which it was separated in the other. The additional terms are made sufficiently clear by the diagram.