34 Mr. Hopkins on the Structure of the 



third (No, 9) a longitudinal section along the axis of the whole district from the Bas 

 Boulonnais to the west of the Wiltshire Downs*. 



r-^i- 



S 8- 



9. 



Bas Boulonnais. Brit. Channel. Sussex. Wiltshire. 



It thus appears that if there had been no denudation, we should have had an 

 elevated range, the base of which would have occupied the whole of the tract de- 

 fined above. The height of different portions of this range would have been very- 

 different. Within the boundary of the Wealden denudation, the greatest elevation 

 above the sea would have been equal to the actual height of the central portion of 

 the district together with the thickness of all the beds superior to the Hastings 

 sand up to the tertiaries inclusive. This would probably have amounted to not 

 less than 4000 feet, while the elevation of the western part of the range would 

 perhaps not have exceeded 2000 feet, being its present height together with the 

 thickness of the tertiaries. 



For the purpose of presenting more distinctly to the mind the actual amount of 

 elevation in this tract, I have supposed the tertiaries and the inferior formations 

 now wanting to have been once continuous over the whole surface of the district. 

 It would manifestly, however, answer our purpose equally well, if, instead of com- 

 paring the elevation of this imaginary surface of the tertiaries with their actual 

 surface in the surrounding country, we should take the surface of any continuous 

 stratum, and compare its elevation within the disturbed district with its depression 

 beneath the actual surface of the surrounding district. The relative elevation of 

 each stratum, real or imaginary, taken with reference to its undisturbed position 

 beyond the bounds of the disturbed district, will be the same, and may be termed 

 the geological elevation of the district, in contradistinction to the actual elevation 

 of its existing surface. 



Taking, then, the whole disturbed tract as above defined, since the geological 

 elevation of the Wealden portion of it, with the Bas Boulonnais, is so much greater 

 than that of the remaining portion generally, it is concluded, in the application of 



* The following are the references to these diagrams : 



a. Tertiaries. e. Hastings sand. 



b. Chalk, Upper greensatid and Gault. f. Kimmeridge clay. 



c. Lower greensand. y. Coral rag. 



d. Weald clay. h. Oxford clay. 



In the Bas Boulonnais the chalk and greensand repose unconformably on the Kimmeridge clay and lower 

 beds. 



