44 Mr. Hopkins on the Structure of the 



tion and fracture might undoubtedly be produced by the expansion of a solid mass 

 immediately beneath the superficial crust, but the phsenoraena of faults, in which 

 the beds on one side of a dislocation are frequently raised to so considerable a 

 height above the corresponding beds on the other, would manifestly not result from 

 this kind of expansion ; and therefore it is concluded that the elevation must have 

 been owing to the expansion of a fluid or gaseous mass, since there appears to be 

 no other conceivable mode of producing the elevatory force. I shall now endea- 

 vour to explain how we may distinctly conceive the requisite mechanical action to 

 have resulted from this agency. 



It will be recollected, that in the investigations by which the effects of elevatory 

 forces have been determined, the elevated mass has been assumed to be flxed at 

 each point of its boundary, — a condition which is manifestly satisfied by the hypo- 

 thesis of an internal cavity, co-extensive with the surface of the elevated district. 

 If the observed phsenomena be such as are referable to an approximately uniform 

 action of the elevatory force, they will only require the hypothesis of a continuous 

 and uninterrupted cavity, occupied by matter possessing the property of fluidity in 

 a very considerable, though not necessarily in a perfect degree. In general, how- 

 ever, the phenomena indicate a more energetic action in particular portions of the 

 disturbed district, and in such cases some additional considerations become neces- 

 sary. For the greater simplicity, I have usually considered these phsenomena as 

 due to a more intense action of the elevatory force, the resistance of the uplifted 

 mass being uniform. This resistance, however, may be different in different por- 

 tions of the mass, and the resulting effect will then depend not merely on the in- 

 tensity of the elevatory force, but on the ratio which that intensity bears to the 

 resistance opposed to it ; so that the effects will be the same, whether the force be 

 variable and the resistance uniform, or the converse. Thus, for instance, the greater 

 geological elevation of the Wealden district as compared with that of the other 

 portion generally of the whole disturbed tract, might be referred to a smaller resist- 

 ance of the uplifted mass ; to account for which, we have only to suppose a certain 

 portion of the superior beds which are now wanting in that district, not to have 

 existed there at the epoch of its elevation. The absence of this superincumbent 

 weight, conjoined with a uniform force throughout the whole disturbed tract, would 

 manifestly be equivalent to the action of a greater force beneath that portion of the 

 district, conjoined with a uniform mass ; and the phsenomena of elevation would in 

 either case be sensibly the same. 



If we would refer a greater relative geological elevation to a greater intensity of 

 the elevatory force, it may be desirable (though not essential to our immediate 

 object, as stated in Art. 3.) to explain how that intensity might be greater in one 

 part of the internal cavity than in another, notwithstanding the tendency of a 



