Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/318

Rh SCH course have the effect of bringing the dislocated ends of the ' left hand. beds against the line of dislocation. In ﬁg. 37, for in- ' 1'10. 37.—Plan of strata traversed by a diminishing strike-fault. stance, which represents in plan another strike -fault, we see that the amount of throw is diminishing towards the left so as to allow lower beds to successively appear, until, at the extreme left side of the ground, the fault merely brings one part of the same bed (No. 5) against another part. 3. Their effects become more complicated where faults traverse undulating and coutortetl strata. Sometimes we ca.n distinctly trace an undulation as the result of a fault. In the ﬂat limestone beds shown in fig. 38, for example, M l r‘ i ll I-‘IG. 38.—-Curving of strata on one side of a fault. there can be no doubt that the gentle depression from (I to c would not have taken place but for the existence of the fault ab. But in all countries where the rocks have been thrown into folds and corrugations these structures are tra- versed by faults. It then often happens that the same fault appears to be alternately a downthrow on opposite sides. Let us suppose a series of gently rolling strata to be cut by a transverse fault as in the diagram in fig. 39. l-'16. 39.—Diagram of gently undulating strata cut by a fault, with alternate throw in opposite directions. At each of the two ridges on the near side of the fault the effect is an upthrow, while in the intervening valley it is a downthrow. On the opposite side of the fault each of these effects is reversed. It rarely happens, however, that a fault makes any such visible crack at the surface. The rocks have all been worn down so much that it is usually only by careful examination of their dip that the existence of faults can be determined. The influence of faults upon curvatures may be illustrated by a plan and sections of a dislocated anticline and syncline, which will also show clearly how the apparently lateral dis-- placement of outcrop produced by dip-faults is due to vertical movement. Fig. 40 represents a plan of strata thrown into an anticlinal fold AA and a synclinal fold SS, and traversed by a fault FF, which is an upthrow to the GEOLOGY [1v. STRUCTURAL. We have seen that a dip-fault always shifts the outcrop to the dip on the upthrow side, and this will be I-" (I 11‘ .1 1--‘ . ‘ --T‘ 4- ‘ ._' V60. ' I ‘ s 5 60' 11 (17 FIG. 40.—An anticline (A) and sync-line (S), dislocated by a fault. observed to be the case here. Beginning at the upper side of the diagram, which may be called north, we notice that the bed aa, dipping towards the lower side or south at 60’, is truncated by the fault at 2:, and that the portion on the upthrow side is shifted forwards or southward. Crossing the syncline we meet with the same bed, and as the upthrow of the fault still continues on the same side we must go some way southwards ou the downthrow side before we meet with its continuation. On the southern slope of the anticline the same bed once more appears, and again is I’ '1-“‘ I I’ ‘ I .  '1 ” A ‘  . W/W xvw so I 1 ‘  Tl

I-‘re. 41.—Section along the upcast side of the fault in fig. -10. shifted forwards as before. A sect.iou along the left or upcast side (uu) of the fault would give the structure re- presented in ﬁg. 41; while one along the dowucast side I’-‘ I I I I  60' 60. a, 60*’: I,’ A ‘ (Z S

FIG. 4'.?.—Scction along the downcast side of same fault. ((141) would be as in fig. 42. These two sections clearl_v prove that the shifting of the outcrops at the surface can 1; cl FIG. 43.—Plan of single fault. be simply explained by a mere vertical movement. They also show that faults which cross anticliual and synclinal