Wealden District and the Bas Boulonnais. 



33 



action here assumed, we may observe that it is exactly that which would be pro- 

 duced on a nearly horizontal surface by the pressure of a fluid, the action of which, 

 being perpendicular to the surface, would be nearly vertical. 



4. In applying this theory to any proposed district, we have first to consider 

 whether the elevatory force (which, according to our fundamental hypothesis, has 

 acted simultaneously at every point of the lower surface of the uplifted mass,) has 

 acted with the same or with a different intensity on different portions of that sur- 

 face. If the force acted under any continuous portion of the mass with a greater 

 intensity than under another such portion, the former would, ceteris paribus, be 

 more elevated and disturbed than the latter ; and assuming the undisturbed posi- 

 tions of the component strata and the external surface to have been originally hori- 

 zontal, the position of any stratum, or that of the surface immediately posterior to 

 its elevation, and anterior to any subsequent subsidence, would indicate these two 

 portions of the elevated mass. In determining them by observation, we must 

 necessarily assume the external configuration of the proposed district, (independ- 

 ently of subsequent denudation), and the disturbed forms of the strata, to retain in 

 a certain degree the characters impressed on them by that decisive movement to 

 which our theory may refer the more characteristic phaenomena of elevation. As 

 an elucidation, let us take the tract of country with which we are here concerned, 

 including the Bas Boulonnais and nearly the whole of that portion of England 

 lying south of the Thames, and extending to the west of the Wiltshire Downs ; 

 and let us assume, for the moment, that every part of this tract was elevated 

 simultaneously ; and for the greater simplicity of the elucidation, we may also 

 suppose it to have received its whole elevation at a single movement. To judge of 

 the relative elevations of different portions of it immediately after this movement, 

 we must consider what would be the form of the external surface at the present 

 time if no denudation had taken place. For this purpose we must conceive the 

 tertiary beds to extend over the whole of this tract (since the elevation took place 

 posterior to their deposition), as represented in the accompanying diagrams. The 



AT 



first (No. 7) represents a general section across the Weald, from north to south ; 

 the second (No. 8) a similar section west of the Wealden denudation ; and the 



VOL. VII. SECOND SERIES. F 



