Structure of the Wealden District, and the Bas Boulonnais. 321 



freshwater fishes hitherto described; but that they nevertheless be- 

 long to an ichthyological fauna, more modern and more nearly re- 

 sembling the recent than any other with which M. Agassiz is ac- 

 quainted in a fossil state. 



Similar remains have been found by Mr. Lyell at Runton, near 

 Cromer, but both there and at Mundesley the associated testacea all 

 belong to living freshwater species ; even the Paludina minuta (Strick- 

 land), which Mr. Morris has pointed out to the author to be iden- 

 tical with the P. marginata of Michaud, a living French species. 

 It is a question therefore, the author states, whether these unknown 

 fishes may not still inhabit the rivers and lakes of the more northern 

 parts of Europe or America, especially as M. Agassiz is at present 

 unacquainted with the freshwater fishes of Norway, Sweden, Spitz - 

 bergen, Iceland, Greenland, Labrador and Canada, and even of the 

 northernmost parts of Scotland and the Shetland Islands ; and in 

 conclusion Mr. Lyell says, it seems natural to look northward for 

 types analogous to the Mundesley fishes, because the beds in which 

 they occur were deposited contemporaneously with the drift accu- 

 mulated by the agency of floating ice. 



Feb. 3. — A paper was read, " On the Geological Structure of the 

 Wealden District, and of the Bas Boulounais," by William Hopkins, 

 Esq., F.G.S. 



This paper is divided into two parts. In the first the author de- 

 scribes the phsenomena of elevation presented in the two districts 

 comprised respectively within the boundary of the great Chalk 

 escarpment of the south-eastern part of England, and an exactly simi- 

 lar escarpment forming the inland boundary of the Bas Boulonnais. 

 The former is well known as extending from the coast at Folkstone, 

 by Seven Oaks, Godstone, Farnham, Petersfield, &c, round to the 

 coast again at Beachy Head. On the opposite side of the channel, 

 the escarpment, commencing at Wisant on the north, forms nearly 

 a semicircle, of which Boulogne is not far from the centre. If we 

 conceive the northern Weald escarpment continued from Folkstone 

 to Wisant, and the southern one from Beachy Head to the southern 

 extremity of the Bas Boulonnais, it will be seen that the whole tract 

 comprised within the Chalk would be a regular oval, except that its 

 axis instead of being straight is curved, so as to incline towards the 

 S.E. in its eastern portion. These two districts are thus connected 

 by relative position not less than by a community of geological cha- 

 racter. 



In the second part of his paper the author compares the laws of 

 the existing phaenomena in these districts with the results given by 

 his ' Theory of Elevation,' published in the Transactions of the Cam- 

 bridge Philosophical Society (Vol. VI. Part I.).* 



I. The lines of elevation in the Wealden district are partly marked 

 by an anticlinal arrangement of the beds, and partly by strong flex- 

 ures, forming one-sided saddles. The latter have been termed by the 



[* Mr. Hopkins gave a view of this subject in Phil. Mag., Third Series, 

 vol. viii. p. 227 — Et'iT.] 



Phil. Mag. S. 3. Vol. 19. No. 124. Oct. 1841. Y 



