Ch. V.] 



FAULTS. 



63 



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

 of disturbed strata, against inferring repeated alternations 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. 8*7) represent the surface of a 

 country on which the strata a b c frequently crop out, an observer, who 



Fie. st 



G- .. 



a ,-•-' 



[■■'%.-■' 



-ic 





e 



" a „-- 









^z^c^* 



Wk 



Apparent alternations of strata caused by vertical faults. 



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 vertical faults, or downthrows. Thus, suppose the origi- 

 nal 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 success- 

 ively, so as to leave vacant the spaces marked in the diagram by dotted 

 lines, and to occupy those marked by the continuous lines ; then let de- 

 nudation take place along the line A H, so that the protruding masses 

 indicated by the fainter lines are swept away, — a miner, who has not dis- 

 covered 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 first on arriving at the fault G he is stopped sud- 

 denly in his workings, upon reaching the strata of sandstone c, or on ar- 

 riving at the line of fault F, he comes partly upon the shale b, and partly 

 on the sandstone c, and on reaching E he is again stopped by a wall com- 

 posed of the rock d. 



The very different levels at which the separated parts of the same strata 

 are found on the different sides of the fissure, in some faults, is truly 

 astonishing. One of the most celebrated in England is that called the 

 ' : ninety-fathom dike," in the coal-field of Newcastle. This name has 

 been given to it, because the same beds are ninety fathoms lower on the 

 northern than they are on the southern side. The fissure has been filled 

 by a body of sand, which is now in the state of sandstone, and is called 

 the dike, which is sometimes very narrow, but in other places more than 

 twenty yards wide.* The walls of the fissure are scored by grooves, such 



* Onybeare and Phillips, Outlines, etc. p. 376. 



