Wealden District and the Bas Boulonnais. 25 



Independently of such evidence as the above, the opinion that river-courses 

 through the chalk escarpment have originated in transverse fissures must rest on 

 analogy, or more especially on the degree in which the hypothesis harmonizes 

 with that general theory which may be recognised as best accounting for the 

 general pheenomena of elevation. I shall again refer to the same point in the con- 

 cluding section of this memoir. 



The Bas Boulonnais. 



The denudation of the Bas Boulonnais has laid bare certain portions of the 

 oolitic series, which, together with the cretaceous beds, were first described by Dr. 

 Fitton and subsequently in greater detail by M. Roget, to whose work I am indebted 

 for much information. I shall find it necessary to repeat in some measure what 

 had been previously observed by these gentlemen respecting the superficial range 

 of the different beds of the district, in order to convey a more distinct conception 

 of its geological structure. 



At Wissant the gault is observed to emerge at a shght inchnation, but is again 

 immediately lost under the accumulations of sand [les dunes), extending nearly two 

 miles along the coast. To the south of these dunes the Kimmeridge clay com- 

 mences and occupies the whole range of coast, with the exception of some other 

 dunes at the mouths of the rivers, till we arrive at those which extend along the 

 coast from the southern part of this district for many miles. In proceeding east- 

 ward from the coast, we find the coral rag and Oxford clay brought to the surface 

 by a very small rise of the strata in that direction. On the south of the river 

 which falls into the sea at Ambleteuse, the Oxford clay is the lowest bed which is 

 thus exhibited (Diagram No. 26.)*. On the north of that river the same clay is 

 brought up in like manner, but by a rather greater inclination of the strata, and 

 consequently at a smaller distance from the coast (Diagram No. 25.). An inferior 

 portion of the oolites also here emerges from beneath the Oxford clay, and reposes 

 immediately and unconformably on a portion of the carboniferous series consisting 

 of mountain limestone and interpolated beds of coal. 



This last circumstance exhibits that great discontinuity in the process of deposi- 

 tion, so generally recognised in this part of the world, which took place after the 

 formation of the carboniferous series. Another important instance of a similar 

 kind is also not less clearly shown in this district than in the southern coast of 

 England, in the entire unconformity of the chalk, gault, and greensand with the 

 beds on which they repose. To the west of the Calais road through Marquise, the 

 lower of these beds appear to come in contact with the Oxford clay. Immediately 

 to the east of that road the mountain limestone presents itself, and to the north of 

 * These diagrams of the Bas Boulonnais are appended to the memoir, and will be found in pages 47, 48. 

 VOL. VII. SECOND SERIES. E 



