Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/320

Rh 306 much more easily raised into the position Ar(‘f, than the mass II into the position Df'Bv’, the elevation of which could hardly take C I) Fig. 50. place without leaving a great open gap along the line of fault between FE and f ’c’, and, moreover, without leaving the projecting piece c’ overhanging without any support. " This is yet more clearly perceptible if we suppose two such ﬁssures, as in fig. 51, inclining towards each other, since, if we sup- is H D Fig. 51. pose the included piece I to be elevated into the position indicated by the dotted lines, it becomes utterly unsupported unless we sup- pose huge dykes or ejections of igneous rock to issue out along each fault. But this would remove the case from the class of fractures we are at present considering.” Trough-faults offer at ﬁrst some diﬂiculty. In ﬁg. 48, for example, it is evident that in both the wedge-shaped masses (A and B) there has been subsidence. The bed X is cut by four faults. In the space B two of these faults hade towards each other, and as they have the same amount of throw the level of the bed remains unchanged on either side. In the other trough, however, the fault a has a throw twice as much as that of b which it completely cuts off. The two faults (Z and f neutralize each other, and are con- nected with a vertical ﬁssure without any throw. The fault a however descends with its persistent hade and dislocates the bed Z and the other strata below. Mr J ukes proposed the following satisfactory explanation of this kind of struc- ture. . “ Suppose the beds AA, BB, &c., (ﬁg. 5:2) to have been formerly in a state of tension, arising from the bulging tendency of an C Fig. 52 internal force, and one fissure, FE, to have been formed below, which on its course to the surface splits into two, El) and EC. If the elevatory force were then continued, the wedge-like piece of rock V between these two ﬁssures, being unsupported, as the rocks on each side separated, would settle down into the gap as in fig. 53. If the elevatory action were greater near the ﬁssure than farther from it, the single ﬁssure below would have 9. tendency to gape upwards, and swallow down the wedge, so that eventually this might settle down, and become ﬁxed at a. point much below its previous relative position. Considerable friction and destruction of the rocks, so as to cut off the corner _r/h (fig. 53) on either side, would probably take place along the sides of the fissures, and thus widen the gap, and allow the wcdgrv-shaped piece V to settle down still further. G E O L O G Y [1v. STRUCTUR.-I.. “ When the forces of elevation were withdrawn, the rocks would doubtless have a tendency to settle down again, but these newly- Fig. 53. included wedge-shaped, and other masses, would no longer lit int - the old spaces, so that Great compression and great lateral f-1'L‘Ssu1".' might then take place. In ﬁg. 49 an excellent illustration is afforded of how an archedxnass of strata has been faulted. and how trough-faults have been formed. VI. -—C'LE_u'.u:t:. There is yet another system of divisional planes, termed cleavage, by which rocks are sometimes traversed. When this structure is well developed it divides a rock into parallel lamime, which run at a high angle quite independ- ently of stratiﬁcation or any other divisional phnes. It is most perfect in proportion to the fineness of grain of the material in which it occurs. Hence ﬁne argillaceous 1‘ucl{s show it admirably. An ordinary rooﬁng slate may be taken as an illustration of a cleaved rock; its opposite surfaces are cleavage-planes, while the opposite faces of a slab of shale would be stratiﬁcation-planes. Though in ».~t perfectly exhibited by clay-slate, cleavage occurs in other rocks, even in old lavas and tuffs, limestones, and sandstones or greywackes ; but as the texture increases in coarseness the cleavage lines become more undecided and further apart. The structure may be observed to vary in distinctness in the same face of rock, being well-deﬁned among bands of slate, but becoming faint or even disappearing in intercalated beds of sandstone or grit. Fm. 5-t.—Sketch (by the late .Ir Du I'u_vcr) of :1 blink of variegated slate from Devil's Glen. county Wiclalmv. 'l'hc crulnplcd l):lllll.'H1:ll'li tltr-‘he(l«lim:, am] the ﬁne perpendicular strite in front are the cleavage planes: the tine lines on the darkened side mcr-:l_v re‘-present sluulmr, and must not be taken for planes of division in the rock. It will be observed that the cleavage planes do not pass through the white bands. The direction of cleavage usually remains persistent ovcr considerable regions, and, as was shown by Sedgwick, corre-