Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/319

Rh DISLOCATIONS or iioci{s.] G E O L O Y 305 folds narrow the anticlines but widen the syneliiies on the l ing ground is displace-:1. The maximum displacement in dowiitlirow side, while they widen the aiiticliiies and such an instance would be sought for towards b; in the narrow the synclines on the uptlirow side. direction e there would be no displacement at all. Dislocation may take place either byasingle fault or as the It often happens that, by a succession of parallel and combined effects of two or more. Where there is only one adjoining faults, a series of strata is so dislocated that a. fault, as in ﬁg. 43, one of its sides may be pushed up or let given stratum which may be near the surface on one side is down, or there may be a simultaneous opposite movement on carri.ed down by a series of steps to some distance below. either side. In such cases, there must be a gradual dying Excellent examples of these step-faults (fig. 47) are to be out of the dislocation towards either end ; and there will 11su- ally be one or more points where the displacement has reached a niaxinium. Sometimes, as shown in ﬁg. 4-1, a fault with a considerable maximum throw (35 feet, yards, or fathoms, in the drawing) splits into minor faults at the terminations. Exaiiiplcs of this kind occur not infrequently in coal-work- G j l 5’ (_ I 2!L ?TJ‘‘-j—Z --1<"‘,_.-__ b, / r (I Fm. 44.—Plun of a fault splitting into iiiiiior faults. Fm‘ 47'_S°cﬁ°n of strata cut by 5t°p'f'mltS' seen in the coal-fields on both sides of the upper part of the estuary of the Forth. Instead, however, of having the same downthrow, parallel faults frequently show a move- ment in opposite directions. If the mass of rock between . them has subsided relatively to the surrounding ground, ,- “’f * 3- they are t2'ou_(/la-faulls (fig. 48). They enclose wedge—shaped masses, of which the apices, formed by the junction of two ings. In o'ther cases the offshoots take place along the line of the main fissure (fig. 45). Exceedingly complicated 7: iv Id ‘/  "‘“"*.:::~'»'~. .~:.'f~’«‘ ""%»==v:;u.a-.:¢””f1n.'A_w7ii/Mlﬂwy/'7“ FIG. 45.——Plan of main fault, with branches. "-“ _ examples occur in some coal-ﬁelds, where the connected - h'.l(,' .l......., ....,..v-. A11. ‘?—~_- faults become so numerous that no one .of them deserves to be called the main or leading dislocation. The subsidence or elevation of a large mass or block of rock has more usually taken place by a combination of ~°‘°°°“5“%==-‘=~“- faults. lf we suppose two ﬁssures to meet at a point-, as at b in fig. 46, and to die out respectively at a and c, the FIG. 4S.—'l‘rough—faults. rt. . . ‘L faults, point downwards. In the accompanying section (fig. 49) of a portion of the thick coal of South Stafford- shire, drawn to scale by Mr Johnson of Dudley (Records qf Gcol. Surya‘:/, vol. i. part 2, p. 313), the commencement of a trougli-faiilt is shown in the centre of the ﬁgure. The late Mr J ukes carefully described this interesting section, and showed that the coal must once have been more arched than now, and that on the cessation of the elevatory process the fractured pieces adjusted themselves to their new position by means of disloca- tions. The iiiass of hi her beds (A) driven as a wedge into the coal, has hindered the bed rom regaining its liorizontalit_v, and at the 5‘ same time has caused the adjacent parts of the coal (BB) to be so Fm 4;;__phm of mo ﬁsSm.es_ crushed by the euornious pressure as to have been reduced to “a _ _ . paste of coal dust and very small coal” (i’l[c7no2'1' on South Staﬂ"or(l- intervening triangular mass ed may be moved upwards or ski-rc Coal-ﬁclcl, 2d ed., p. 194). downwards, or it may remain stationary while the surrouiid- It will be observed that the hade of the faults is towards Fic. 49.—<cction of a faulted part of the thick coat of South Staffordshirc. the downthrow side, and that the wedged-shaped masses “S11PP0Se,"11e says, “ that in diagram fig. 50 we have a portion with broad bottoms have risen, while those with narrow Of the eartllis °“‘St'.°f “'1‘i"h AB is the S“‘f‘.‘°°7 ‘"“l.('D 3 Plane bottoms and broad tops have Sunk acted on by soiue widespread force of expansion tending to bulge _ upwards the part ABCI). If then a fracture takes place ﬂl0ll0‘ the It has been already (cmte, p. 2361) pointed out that faults line EF, it is obvious that the expanding force will, on the side of are traceable to the effects of elevation The general hade AC, have the widest base CF to act upon, while it will have a pro. or inclination of faults towards the side of downthrow was I’°"l1i°“1"1‘te1Y If” "1335 :0 1‘“°"° fl“ “"13 1”“ A1ECFi1“'1‘l‘:1h 3f"‘3[;‘S - . - - V _ P‘ . crrac ua y sina ier towart s t ie sur ace t iaii on t ie ot er si e 0 ie satisfactorily explamed b3 the late Mr Ju1‘e° 1“ the last fault, where, with the smaller base F l), the mass F DBE contimiall_v edition of the Present W0"kv grows larger towards the surface. The mass G will consequently he X- — 39