YALLEYS OP DISLOCATION. 153 



mass of the same bed of which they once formed a part. 

 Thus, in the accompanying figure 90, the beds a and b form 

 outliers of the main strata, c and d; the missing portion 

 having been removed by denudation, and their identity being 

 proved by the accordance of mineral character, position, and 

 imbedded fossils. 



ESCARPMENT. Strata are said to form an escarpment 

 when they terminate abruptly, as at a, 5, in the above 

 figure 90. A mural escarpment is one of a steeper and 

 more perpendicular character. 



OBIGIN or VALLEYS. When the opinions of Werner 

 more generally prevailed, and water was regarded as the 

 universal agent in producing geological phenomena, many 

 effects were attributed to aqueous agency which are now 

 ascertained to have been occasioned by other causes. It was 

 then usual to refer all valleys to aqueous action, and to 

 consider them as having been produced by the rivers and 

 streams which flow through them. It soon, however, became 

 evident that this cause alone was insufficient, and that in 

 many instances they had been produced by other agencies ; 

 these conclusions being founded chiefly on the fact, that 

 many streams, instead of running through soft strata, avoid 

 . these, and find their way through rocks of harder texture. 

 It thus became obvious, that the cause is to be sought rather 

 in the rocks themselves, than in the waters which flow over 

 them. Valleys are now divided into four classes, instead of 

 being limited to that of aqueous origin. 



VALLEYS OP DISLOCATION. These are caused, by fissures 

 of various dimensions, some of colossal size, which have been 



FIG. 91 



formed during the upheaval and separation of the strata in 

 the region of which they form a part. They usually present 

 steep escarpments, the strata on each side frequently bearing 

 evidence of former continuity. The mode by which they 

 have been produced will be perceived by the accompanying 

 figure 91. 



