20 



GEOLOGY. 



A series of illustrations will make clear the problems involved; but to under- 

 stand them, it should be recalled that the general structure of the scries is mono- 

 clinal, with the general dip to the cast. If the dip were always to the east (or 

 in any constant direction), faults parallel to the strike (strike faults) would pro- 

 duce one series of phenomena, faults at right angles to the strike (dip faults, 

 CD, Fig. 325) would produce another series, and faults oblique to the strike 



Fig. 325. Fig. 326. 



Fig. 325. — Diagram showing the position of dip faults, oblique faults, etc. The 

 black band represents the outcrop of a layer of rock on a plane surface, and there- 

 fore the strike of the rock. AB= the direction of a strike fault, CD the direction 

 of a dip fault, and GH and EF directions of oblique faults. 



Fig. 326. — Diagrammatic section showing dipping beds. 



(GH and EF, Fig. 325) would produce still another (see also Vol. I, pp. 521 

 and 525). 



1. Suppose a series of sedimentary beds with constant dip to the east to have 

 a single trap sheet, t, interbedded (Fig. 326). Suppose the series to be affected 

 by a strike fault with upthrow to the east. After erosion has cut down the up- 

 throw side to the level of the other, any layer (say the trap) will outcrop in two 

 parallel belts (t, Fig. 327). Had the upthrow been to the west (the dip being 

 east) a repetition might not have occurred, and the outcrop of a given la} r er, 

 such as the trap, might have been eliminated. If the faulted surface had been 

 reduced to the level of AB (Fig. 328), the trap sheet would not have appeared 

 at the surface. 



2. Assume the same series of beds to be affected by a dip fault (outcrop of 

 fault plane along CD, Fig. 325) with the upthrow to the south. After erosion 

 has brought the upthrow side to the level of the other, the layer of trap will out- 

 crop in the manner shown in Fig. 329. If the upthrow had been to the north, 

 the result would have been as shown in Fig. 330; that is, in the case of a dip fault, 

 the outcrop of a layer on the upthrow side is (after erosion) shifted in the direc- 



