PART I. CHAPTER V. 



77 



Faults. 



failure of support. Sometimes, however, even these small slips 

 may have been produced during earthquakes ; for land has been 

 moved, and its level, relatively to the sea, considerably altered, 

 within the period when much of the alluvial sand and gravel 

 now covering the surface of continents was deposited. 



I have already stated that a geologist must be on his guard, in 

 a region of disturbed strata, against inferring repeated alterna- 

 tions of rocks, when in fact, the same strata, once continuous, 

 have been bent round so as to recur in the same section, and 

 with the same dip. A similar mistake has often been occasioned 

 by a series of faults. 



If, for example, the dark line A H (Fig. 78.) represent the 



Fig. 78. 



Apparent alternations of strata caused by vertical faults. 



surface of a country on which the strata a b c frequently crop 

 out, an observer, who is proceeding from H to A, might at first 

 imagine that at every step he was approaching new strata, 

 whereas the repetition of the same beds has been caused by ver- 

 tical faults, or downthrows. Thus, suppose the original mass, 

 A, B, C, D, to have been a set of uniformly inclined strata, and 

 that the different masses under E F, F G, and G D, sank down 

 successively, so as to leave vacant the spaces marked in the dia- 

 gram by dotted lines, and to occupy those marked by the conti- 

 nuous fainter lines, then let denudation take place along the line 

 A H, so that the protruding and triangular masses indicated by 

 the fainter lines are swept away, a miner, who has not discov- 

 ered the faults, finding the mass a, which we will suppose to be 

 a bed of coal four times repeated, might hope to find four beds, 

 workable to an indefinite depth, but on arriving at the fault G he 

 is stopped suddenly in his workings, upon reaching the strata of 

 sandstone c, or on arriving at the line of fault F he comes partly 

 upon the shale 6, and partly on the sandstone c, and on reach- 

 ing E he is again stopped by a wall composed of the rock d. 



