APPENDIX. 



195 



(Fig. 22) given by Mr. Rogers exhibits the facts last mentioned. In the 



matter of fractures and faults, i. e., in respect of the tension in which 



these cracks originate, each anticlinal belt may to some extent be looked 



upon as a distinct area of elevation ;. each wave has its own system of 



fissures. In this way Mr. Rogers subordinates the fissures, at least the 



lateral fissures of a great system, to the flexure ; he says, " these great 



fractures are only flexures of the more compressed type which have 



given way in the act of bending." In the great majority of instances 



these fissures coincide neither with the anticlinal nor the synclinal 



axis planes,* but with the steep or inverted sides of the flexures. 



He describes such a fault eighty miles long, and having a throw of 



8,000 feet. Faults such as are here described generally underlie in 



the same direction as the axis-plane of the flexure, and it very 



commonly happens that the upthrow takes place on the side of the 



underlie, producing, as normal results, what are commonly called reverse 



faults. Fig. 23. 



Fig. 23 represents an anticlinal 

 flexure so faulted. Faults of this 

 kind are, as their name indicates, 

 generally supposed to occur very 

 l'arely, and only on a small scale. 

 Mr. Rogers, however, describes them 

 as a common feature in mountain 

 structure. They produce the ano- 

 malous appearance of older strata 

 overlying newer. The folded flexure 

 itself produces this effect, but in a 

 less marked manner than when 

 faulted. 



It was only the absence of any ascertained order in the phenomena of 

 contortions that has hitherto excluded them from their due position 

 in speculations upon the nature of the causes of disturbance of the 

 earth's crust. In the comparatively regular structure of the Alleghany 

 mountains, Mr. Rogers conceives that he has discovered this order. He 

 asserts that undulation is the prevailing law of all displaced deposits, 

 and that waves of translation are the archetypes of these undulations. 

 He consequently declares that any theory upon this subject, henceforth 

 admissible into physical geology, must explain the general facts of 

 the regular wave-like structure of the earth's disturbed zones. He 

 points out that no simple upward pressure could have this effect. He 

 affirms that simple lateral pressure could not result in such regularity. 

 He conceives this structure to have originated in a true wave 



b& a 



Reverse fault along a folded anticlinal 

 flexure. — (Rogers . ) 



* The axis-piano of a flexure is a plane bisecting the angle of incurvation. 



