Wealden District and the Bas Boulonnais. 31 



by the carboniferous and older formations. An account of its structure is in some 

 measure foreign to my present object, since it must be ahnost entirely independent 

 of the elevatory movement which led to the denudation of the district. I shall 

 only offer one or two remarks on the subject. The surface of this portion of the 

 district forms a rather elevated plateau bounded by the chalk hills on the north, 

 and by the river which runs by Marquise on the south. On the north side the 

 beds dip very rapidly to the south. The surface of the southern boundary is covered 

 up by a thin covering of the unconformable oolites, but in one part I found a clear 

 section of the inferior beds, which showed them dipping very rapidly and regularly 

 to the north. If such be the case along the whole southern boundary (as I think 

 extremely probable) , the central portion of these older formations must lie in a 

 synclinal trough. The dip is there very irregular, and generally much less than 

 on the northern boundary, or at the point where I observed it on the southern limit. 

 These facts are easily accounted for by the structure now described. 



§ 2. Theory of the Elevation of the District. 



The special object of this second section of my memoir is the comparison of the 

 phsenomena above described with the conclusions drawn from the theory of eleva- 

 tion as given in the Transactions of the Cambridge Philosophical Society ; but I 

 shall also avail myself of this opportunity of developing several points of that theory 

 more in detail, and of offering such elucidations of the whole subject as the example 

 before us may afford. In doing this I shall add somewhat to the mechanical rea- 

 soning hitherto employed ; but, generally, those parts of the memoir in which we 

 are not engaged in the immediate application of one theory to the district before 

 us, will consist rather of more detailed explanations of fundamental hypotheses and 

 final results, than of the mechanical reasoning, for which I must refer the reader 

 to the memoir just alluded to. 



1, In any accurate researches on this subject, based on mechanical principles, 

 some hypotheses must necessarily be made respecting the constitution of the elevated 

 mass and the action of the elevatory force. In the memoir above referred to, I have 

 obtained the general results, assuming, for the greater simphcity, the cohesive 

 power of the mass to be uniform throughout, and have then shown in what other 

 cases those results would still be accurately or approximately true. The cohesion 

 may have varied continuously according to any law and in any degree, in passing 

 from one point of the mass to another ; or it may have varied discontinuously 

 along any vertical line within the mass, as for instance, in passing from one hori- 



