Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/316

302 limestone bands and soft shales, or where its course has been undulating, the relative shifting of the two sides has occa- sionally brought opposite prominences together so as to leave wider interspaces, as in ﬁg. 27. The actual breadth Fm. ‘27.—Scction of fault, showing the alternate expansions and contractions due to the shifting of one side of a sinuous ﬁssure. of a fault may vary from a mere chiuk into which the point of a knife could hardly be inserted up to a band of broken rock many yards wide. But in these latter cases we may usually suspect that so great a breadth of fractured materials has been produced not by a single fault but by a series of closely adjoining and parallel faults. Faults are sometimes vertical, but are generally inclined. The largest faults, that is, those which have the greatest ‘ —.—.q,_,.,~._.__e‘-- /‘ 7)/).'-Q-éytd s'uo WV ‘ b ' ‘F/*"|1...__ FIG. 28.—Section of a vertical and_inc1ined fault. vertical displacement, slope at high angles, wl1ile those of only a few feet or yards may be inclined as low as 18° or 20°. The inclination of a fault from the vertical is called its hade. In ﬁg. 28, for example, the fault between A and C being vertical has no hade, but that between C and B hades at an angle of 70° from the vertical to the right hand. The amount of displacement is represented as the same in both instances, so that the level of the bed a is raised between the two faults at C above the uniform horizon which it retains beyond them. That faults are vertical displacements of parts of the earth’s crust is most clearly shown when they traverse stratiﬁed rocks, for the regular lines of bedding and the originally ﬂat position of these rocks afford a measure of the disturbance. Accordingly we may consider here the effects of faults as they traverse (1) horizontal, (2) inclined, or (3) undulating strata. 1. In the above section (ﬁg. 28) two faults are supposed to traverse a set of horizontal strata, and to displace them in opposite directions. Hence the portion between them appears as if it had been pushed up, or as if the part on either side had slipped down. The amount of vertical dis- placement is measured from the end of any given stratum, say a, on one side of the fault, to its corresponding end on the other side. Suppose, for example, that the black band in ﬁg. 29 represents a known stratum such as a seam of coal, which, having been explored in underground operations, is known to be cut by a fault at a depth of a hundred yards below the surface at A, and to lie 200 yards deep on the other side of the fault below B. The amount of displacement is the vertical distance between the two severed ends a and b. This is termed the throw of a fault. From these two sections (ﬁgs. 28 and 29) we see that the horizontal distance to which the two ends of a faulted stratum may be separated does not GEOLOGY [1v. STRUCTURAL. depend upon the amount of throw but upon the angle of the hade. In the left-hand fault in fig. 28 there is no hade,

for the fault is vertical; consequently there "is no lateral displacement. In ﬁg. 29, however, where the fault hades considerably, there is a lateral shift of the bed, the end a. being 150 yards to the left of 6. In this example the lateral shift is half as much again as the vertical. It is obvious that a fault of this kind must seriously affect the value of a coal-ﬁeld; for while the coal—sean1 might‘ be worked up to a on the one side and to 6 on the other, there would be a space of 150 yards of barren ground between these two points where the seam never could be found. The lower the angle of hade the greater the breadth of such barren ground. Hence the more nearly vertical the lines of fault, tl1e better for the coal—ﬁelds. In the vast majority of cases faults hade in the direction of downthrow, in other words, they slope away from the side which has risen. Consequently the mere inspection of a fault in any natural or artiﬁcial section suffices in most cases to show which side has been elevated. In mining operations the knowledge of this rule is invaluable, for it decides whether a coal seam, dislocated by a fault, is to -be sought for by going up or down. In ﬁg. 29, for example, a miner working fro111 the right and meeting with the fault at b, would know from its hading towards him that he n1ust ascend to ﬁnd the coal. On the other hand were he to work from the left and catch the fault at a, he would see that it would be necessary to descend. According to this rule a normal fault never brings one part of a bed below another part, so as to be capable of being pierced twice by the same vertical shaft. Exceptional cases, however, where the hade is reversed, do occasionally appear. In ﬁg. 30 a series of strata, 1 to 11, are represented as folded in an inverted anticline, and broken through by a fault along the axis, the portion on the right side having been pushed up. __——-—..—-_ ‘-.~ _ —_~ I-‘la. 30.—Inverte(l anticline and reversed fault. The effect of the movement has been to make the ends of the beds on that side overlie higher beds on the other side. A shaft would thus pierce the same stratum twice. In- stances of reversed faults are chieﬂy met with in much dis- turbed districts, such as mountain chains, where the rocks have been affected by great undulations and corrugations. But instances on a small scale, like that in ﬁg. 31, may now and then be encountered even in lowland districts, where no great disturbance has taken place. 2. Faults traversing inclined strata usually group them-