Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/317

Rh DISLOCATIONS or r.ocKs.] selves into two series, one running in the same general direction as the dip of the strata, the other approximating »,N,,v‘f~r_~,-5 _7-..-.:;_{- --r;»,.a’. - - to the trenzl of the strike. They are accordingly classiﬁed as tZipfaul_/s and strz'/.'e;fu2z7ts. They are not always to be sharply marked off from each other, for the dip-faults will often be observed to deviate considerably from the normal direction of dip, and the strike-faults from the prevalent
 * «tl'll{0, so that in such cases they pass into each other.

A dip-fault produces at the surface the effect of a lateral shift of the strata. This effect increases in proportion as the angle of dip lessens. It ceases altogether when the beds are vertical. Fig. 32 may be taken as a plan of a dip-fault FIG. 32.—Plan of strata cut by a dip-fault. traversing a series of strata which dip northwards at 525°. The beds on the east side look as if they had been pushed horizontally southwards. That this apparent horizontal displacement is due really to_a vertical movement, and to the subsequent planing down of the surface by denuding agents, will be clear if we consider what must be the effect of the vertical ascent or descent of the inclined beds on one side of a dislocation. Take the bed a in ﬁg. 32, and suppose it to be still unbroken by the fault. It will then run in a straight east and west line. When the fault takes place, the part on the west side is pushed up, or, what comes to the same, that on the east side is let down. A horizontal plane cutting the dislocated stratum will show the portion on the west side lying to the north of that on the east side of the fracture. The effect of denudation has usually been practically to produce such a plane, and thus to exhibit an apparently lateral shift. This surface displace- ment has been termed the heave of a fault. Its dependence upon the angle of slip of the strata may be seen by a com- parison of ﬁgs. 33 and 34. In the former ﬁgure the bed a, once prolonged above the present surface (marked by the horizontal line), is represented as having dropped from db to er, the angle of inclination being 25°. The heave amounts to the horizontal distance between I» a11d e. But if the angle should rise to 60°, as in ﬁg. 34, though the amount of throw or vertical displacement remains the same, we see that the heave or horizontal shift diminishes to about a quarter of what it is in fig. 33. This diminu- GEOLOGY 303 tion would continue with every increase of inclination in the strata till among vertical beds there would be no heave at all. ,. J ‘ ‘~ I '-H JVOI't7:. '3 F10. 33.-—SeL-tion along the line of a fault in strata dipping at 2-3'. Strike-faults, where they exactly coincide with the strike, may sometimes remove the outcrop of some strata by never Snuff: JTOTT 7t F10. 34.—Seetion along the line of a fault in strata dipping at 60°. allowing them to reach the surface. Fig. 35 shows a plan of one of these faults (FF), having a downthrow to the north. In crossing the ground from north to south we pass successively over the edges of all the beds, except Nos. 3 }‘;(_ 3;;_-1>1;m of a strike-fault. and 4, which are cut out by the fault as shown in.ﬁg. 36, which is a section drawn across the ground at a right angle to the strike. It seldom happens, however, that such strict coincidence between faults and strike continues for I-‘ic. 3G.—Section across the plan. ﬁg. 35. more than a short distance. The dip is apt to vary a little even among comparatively undisturbed strata, and every such variation causes the strike to undulate and thus to_be cut more or less obliquely by the line of dislocation, which may nevertheless run quite straight. Moreover, any In- crease or diminution in the throw of a strike--fault will of