DISLOCATIONS OF STRATA. 109 



it with the same minuteness through its more hypothe- 

 tical progress. The strata present marks of elevation 

 and dislocation; it is shown that these appearances 

 are visibly produced hy the intrusion of unstratified 

 rocks; it is inferred that they may he produced hy 

 the same causes in places where the agents are invi- 

 sible. It is easy for the reader to complete this pro- 

 cess; or, if he is dissatisfied with the consequences 

 that would arise from thus pursuing it, to withhold 

 his further assent. It is incumbent on an author to 

 place the premises on which a theory may be esta- 

 blished, in as clear a view and on as firm a foundation as 

 his capacity and knowledge permit ; but it is not neces- 

 sary that he should insist on that assent which he 

 may himself have given, as long as one step of the 

 process remains imperfect or one solid objection can 

 be urged. 



In terminating tbis chapter, it is yet necessary to 

 say, that some geologists have suggested the subsi- 

 dence of the strata into subterraneous caverns as 

 causes of their elevation and dislocation. After all 

 that has been urged against the power of such acci- 

 dents to produce the effects of this nature that are 

 visible, I know not that there is more than one variety 

 which it is really incapable of explaining. The force 

 of the gravity of such masses may be conceived equal 

 to any forces of protrusion and expansion that can be 

 assigned ; while, excepting that just alluded to, there is 

 no modification of the supposed direction of the pro- 

 truding force, nor any combination of these, which 

 may not be as adequately represented by the reverse 

 powers of gravitation. This is the case in which 

 masses of strata, often involving large tracts of 

 country, are moved horizontally ; a circumstance of 

 frequent occurrence, and clearly proved by the intru- 



