
Py |. > FAULTS. * 629 
of all the beds, except the part between the asterisks, which is eut 
out by the fault as shown in the section. It seldom happens, how- 
ever, that such strict coincidence between faults and strike continues 
for more than a short distance. The direction of dip is apt to vary 
a little even among comparatively undisturbed strata, every such 
variation causing 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 increase or diminution in the throw 
of a strike-fault will, of course, have the effect of bringing the dis- 
- located ends of the beds against the line of dislocation. In Fig. 259, 
for instance, which represents in plan another strike-fault (/), we see 
' that the amount of throw increases towards the right so as to allow 
lower beds successively to appear on one side, while towards the left 
_ it diminishes, and finally dies out in bed Y. 

























SSS ce 7 





LSS oOo - ee 
SS Wy, I are ee cf 
cy 2 a0 5 See = = E = : Ll Tr — 
20 eee “o ed = ———_-— 
= oO. od ReaD > : 
—S Lc 6? 60 
——— Gi ead bine 
ee é 


Fic. 259.—PLAn oF STRATA TRAVERSED BY A DIMINISHING STRIKE-FAULT, 
Their effects become more complicated where faults traverse 
undulating and contorted strata. The connection between folding 
and fracture has already been adverted to in the case of monoclinal 
bends. It sometimes happens that the plications are subsequently 
fractured so that the fault may appear to be alternately a down- 
throw on opposite sides, according to the position of the arches and 
_ troughs which it crosses. ‘This structure may be illustrated by a 
plan and sections of a dislocated anticline and syncline, which will 
also show clearly how the apparently lateral displacement of outcrop 
produced by dip-faults is due to vertical movement. Fig. 260 
represents a plan of strata thrown into an anticlinal fold AA and a 
synclinal fold SS, and traversed by a fault FF, having an upthrow 
_ (wu) tothe east. A dip-fault shifts the outcrop towards the dip on the 
_ upthrow side, and this will be observed to be the case here. On the 
west side of the fault, the black bed a, dipping towards the south, 
is truncated by the fault at uw, and the portion on the upthrow 
side is shifted forwards or southward. Crossing the syncline we 
meet with the same bed rising with a contrary dip, and as the up- 
throw of the fault still continues on the same side the portion of the 
_ bed on the west side of the fault must be sought further south. 
_ The effect of the fault on the syncline is to widen the distance 
between the two opposite outcrops of a bed on the downthrow side, 
_ or to narrow it on the upthrow side. On the southern slope of the 
anticline A the same bed once more appears, and again is shifted 
2M 

