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. [Illustration: 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