The system of the fractures.—In referring to experiments made upon the fracture of solid blocks under compression (p. 41), it was shown that two series of parallel fractures develop perpendicular to each free surface of the block, and that these series are each of them inclined by half of a right angle to the direction of compression, and thus perpendicular to each other. The fragments into which a block with one free surface would thus tend to be divided should be square prisms perpendicular to the free surface. It would be interesting, if it were practicable, to learn from experiment how these prisms would be further fractured by a continuation of the compression. From mechanical considerations involving the resolution of forces with reference to the ready-formed fractures, it seems probable that the next series of fractures to form would bisect the angles of the first double series or set. Wherever rocks are found exposed in their original attitudes, they are, in fact, seen to be intersected by two parallel series of fractures which are perpendicular to the earth’s surface and to each other and are described as joints. In many cases more than two series of such fractures are found, yet even in these cases two more perfectly developed series are prominent and almost exactly perpendicular to each other as well as to the earth’s surface. This omnipresent double series or set of joints is the well-known set of master joints, and very often it is found developed practically alone (Fig. 36). Over large areas, the direction of the set of master joints may remain practically constant, or this set may quite suddenly give place to a similar set which is, however, turned through half a right angle from the first (Fig. 37). Not infrequently two such sets of master joints are found together bisecting each other’s angles within the same rocks, and to them are sometimes added additional though less perfect series of joint planes.

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.

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.

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.

Methods of detecting faults.—The first effect of a fault is usually to produce a crack at the surface of the earth; and, provided there is a vertical displacement or throw, an escarpment which rises upon the upthrown side of the fault. In general it may be said that escarpments which appear at the earth’s surface as plane surfaces probably represent planes of fracture, though not necessarily planes of faulting. In many cases the actual displacements lie buried under loose rock débris near to and paralleling the escarpment, and in some cases as a result of the erosional processes working upon alternately hard and soft layers of rock, the escarpment may later appear upon the downthrown side or limb of the fault (Fig. 43). As an illustration of a fault escarpment, the façade of El Capitan and many other rock faces of the Yosemite valley may be instanced.

The most satisfactory evidence of a fault is the discovery of a rock bed which may be easily identified, and which is actually seen displaced on a plane of fracture which intersects it (Fig. 42, p. 59). When such an easily recognizable layer is not to be found, the plane of displacement may perhaps be discovered as a narrow zone composed of angular fragments of the rock cemented together by minerals which form out of solution in water. Such a fractured rock zone which follows a plane of faulting is a fault breccia. If the fault breccia, or vein rock, is much stronger than the rock on either side, it may eventually stand in relief at the surface like a dike or wall. At other times the displacement produces little fracture of the walls, but they slide over each other in such a manner as to yield either a smoothly corrugated or an evenly polished surface which is described as “slickensides.” It may be, however, that during the movement either one or both of the walls have “dragged”, and so are curled back in the immediate neighborhood of the fault plane (Fig. 44).

When, as is quite generally the case, the actual plane of displacement of a fault is not open to inspection, the movement may be proven by the observation of abrupt, as contrasted with gradual, changes in the strikes and dips of neighboring exposures (Fig. 45); or by noting that some easily recognized formation has been sharply offset in its outcrops (Fig. 46).

There are in addition many indications rather than proofs of the presence of faults, which must be taken account of in every general study of the geology of a district. Thus the outcrops of all neighboring formations may terminate abruptly upon a straight line which intersects all alike. Deep-seated fissure springs may be aligned in a striking manner, and so indicate the course of a prominent fracture, though not necessarily of a fault. Much the same may be said of the dikes of cooled magma which have been injected along preëxisting fractures.

The base of the geological map.—Modern topographic maps form an important part of the library of the serious student of physiography; they are the gazetteer of this branch of science. Every civilized nation has to-day either completed a topographic atlas of its territory, or it is vigorously prosecuting a survey to furnish maps which represent the relief with some detail, and publishing the results in the form of an atlas of quadrangles. Thus a relief map will erelong be obtainable of any part of the civilized world, and may be purchased in separate sections. Nowhere is this work being taken up with greater vigor than in the United States, where a vast domain representing every type of topographic peculiarity is being attacked from many centers. Here and elsewhere the relief of the land is being expressed by so-called contours or lines of equal altitude upon the earth’s surface. It is as though a series of horizontal planes, separated by uniform intervals of 20 or 40 or 100 feet, had been made to intersect the surface, and the intersection curves, after consecutive numeration, had been dropped into a single plane for printing.

The colored areas upon the geological map are each supposed to indicate where a certain rock type or formation lies immediately below the surface, and this distribution represents the best judgment of the geologist who, after a study of the district, has prepared the map. Unfortunately the conventions in use are such that his observation and his theory have been hopelessly intermingled in the finished product. Armed with the geological map, the student who visits the district finds spread out before him, it may be, a landscape of hill and valley, of green forest and brown farming land, which is as different as may be from the colored puzzle which he holds in his hand. Hidden under the farm vegetation or masked by the woods are scattered outcroppings of rock which have been the basis of the geologist’s judgment in preparing the map. Experience shows that in order to bridge the wide gap between the geology in the landscape and the patches of color upon the map something more than mere examination of the colored sheet is necessary. We shall therefore describe, with the aid of laboratory models, the various stages necessary to the preparation of a geological map, and every student should be advised to follow this by practical study of some small area where rocks are found in outcrop.

Laboratory models for the study of geological maps.—In order to represent in the laboratory the disposition of rock outcrops in the field, special laboratory tables are prepared with removable covers and with fixed tops, which are divided into squares numbered like the township sections of the national domain (Fig. 47). To represent the rock outcrops, blocks are prepared which may be fixed in any desired position by fitting a pin into a small augur hole bored through the table. The outcrop blocks for the sedimentary rock types are so constructed as to show the strike and dip of the beds. (See Appendix D.)

The method of preparing the map.—To prepare the map, use is made of a geological compass with clinometer attachment, a protractor, and a map base divided into sections like the top of the table, and on the scale of one inch to the foot. Each exposure represented upon the table is “visited” and then located upon the base map in its proper position and attitude. The result is the field map (Fig. 47), which thus represents the facts only, unless there have been uncertainties in the correlation of exposures or in determining the position of the bedding plane.

To prepare the areal geological map from the field map, it is first necessary to fix the boundaries which separate formations at the surface; and now perhaps for the first time it is realized how large an element of uncertainty may enter if the exposures were widely separated. It is clear that no two persons will draw these lines in the same positions throughout, though certain portions of them—where the facts are more nearly adequate—may correspond. In Fig. 48 is represented the areal geological map constructed from the field map, with the doubtful area at one side left blank.

Some conclusions from this map may now be profitably considered. The complexly folded sandstone formation at the left of the map appears as the oldest member represented, since its area has been cut through by the intrusive granite which does not intrude other formations, and is unconformably overlaid by the limestone and its basal layer of conglomerate. The limestone in turn is unconformably overlaid by the merely tilted sandstone beds at the right of the map. These three sedimentary formations clearly represent decreasing amounts of close folding, from which it is clear that each earlier formation has passed through an episode not shared by that of next younger age. Of the other intrusive rocks, the dike of porphyry is younger than all the other formations, with the possible exception of the upper sandstone. Offsetting of the formations has disclosed the course of a fault, and from its relations to the dikes we may learn that of these the porphyry is younger and the basalt older than the date of the faulting.

The dashed lines upon the map (AB and CD) have been selected as appropriate lines along which to construct geological sections (Fig. 48, below map), and from these sections the exposed thicknesses of the different formations may be calculated. In one instance only, that of the conglomerate, can we be sure that this exposed thickness measures the entire formation.

Fold versus fault topography.—The more resistant or “stronger” rock beds, as regards attacks of the atmosphere, in the course of time come to stand in relief, separated by depressions which overlie the “weaker” formations. Simple open folds which are not plunging exercise an influence upon topography by producing generally long and straight ridges. More complex flexures, since they generally plunge, make themselves apparent by features which in the map are represented by curves. Fracture structures, and especially block displacements, are differentiated from these curving features by the dominance of straight or nearly rectilinear lines upon the map. The effect of erosion is to reduce the asperity of features and to mold them with flowing curves. The fracture structures are for this reason much more likely to be overlooked, and if they are not to elude the observer, they must be sought out with care. Fold and fracture structures may both be revealed upon the same map.