Wealden District and the Bas Boulonnais. 45 



fluid to exert an equal pressure in all directions. ' For this purpose we may first 

 suppose the fluidity of the matter contained in the cavity to have been imperfect, 

 like that, for example, of newly ejected lava ; or, secondly, we may suppose the 

 communication between one continuous portion of the cavity and the contiguous 

 portions not to have been perfectly free, but partially impeded by solid masses, 

 capable of resisting fusion at the temperature of the surrounding matter. The 

 whole general cavity might thus consist of several portions, with obstructed com- 

 munications, like a number of lakes connected by comparatively narrow or shallow 

 channels. Under such conditions, if any cause of expansion were to act with 

 greater intensity on the fluid contained within one of these internal lakes, the fluid 

 pressure throughout it might be approximately uniform, and much greater than 

 that transmitted by the imperfect fluid through impeded channels to the surround- 

 ing lakes. We should thus obtain that kind of action of the elevatory force which, 

 as above shown, the phsenomena of the Weald require for their explanation*. 



-12. Other suppositions respecting the structure of internal cavities might also 

 enable us to account for the existence of a fluid pressure, acting with different 

 intensities on different portions of the elevated mass. It is not, however, my object 

 at present to enter further into speculations of this nature ; nor is it here necessary 

 for us to consider to which of these hypotheses the greatest probability of truth 

 may attach, or whether the greater observed elevation of such a district as the 

 Weald may have been due to the more intense action of the elevatory force, or 

 the greater weakness of that portion of the elevated crust. It is enough to have 

 shown that the hypothesis of internal cavities does enable us to assign a distinct 

 and simple cause for the mechanical action necessary to account for such phaeno- 

 mena of elevation as we find in the Weald and in other analogous districts. 



I shall not, by here touching upon other points of our theory, lengthen a com- 

 munication which has already exceeded the limits I had intended to prescribe to it. 

 This has arisen from an anxiety to avail myself of the occasion afforded by the 

 admirable example before us, to elucidate the fundamental points and some of the 

 more important results of the theory in its application to the Weald and all similar 

 districts. I have thus hoped also to correct some imperfect, if not erroneous con- 

 ceptions which have been entertained on this subject, whether they may have arisen 

 from an imperfect development of my own views, on my part, or an imperfect 

 comprehension of the conventional meaning of the language I have used, on the 

 part of the reader. Such conventional language must necessarily apj^ertain to every 

 branch of science, nor can it be perfectly understood without a clear knowledge of, 

 at least, the fundamental principles and simpler applications of the science to which 



* It appears to me that the existence of internal lakes with obstructed communications affords the best 

 explanation of the phsenomena connected with earthquakes and volcanic action in South America. 



