PRESTWICH BRITISH AND FOREIGN TERTIARIES. 211 



*Lits coquilliers;' — that it synchronises with some of the older por- 

 tions of the Sables inferieurs" (of M. D' Archiac) : — a view in which I 

 am now confirmed so far as regards the higher antiquity of the London 

 Clay, but which deposit I however now consider not to be exactly re- 

 presented by any synchronous strata in the Paris district. 



This instance of the intercalation of a large and important deposit in 

 England, where in France the sequence of the Lower Tertiaries is so 

 well maintained that there is no appearance of any link missing, is a 

 very remarkable one. Both M. D' Archiac and M. Graves have, as 

 mentioned above, particularly noticed the perfect sequence of the 

 beds from the chalk to the Calcaire grossier, and I can bear testimony 

 to the same fact. Lithological structure and superposition seem to 

 indicate a complete and perfect series, whilst it would appear that 

 the organic remains have not been considered to present any suffi- 

 cient differences to militate against this view. It would nevertheless 

 seem that there is a very important interval between the '' Lignites 

 of the Soissonnais" and the " Lits coquilliers," and that, at so short 

 a distance as from Kent to the Department of the Oise, there is 

 introduced, wedge-shaped, between these two deposits, the large 

 mass of the London clay with its multitude of original organic re- 

 mains. Yet there is not only no evidence either of the great lapse 

 of time, or of the important physical changes which such a formation 

 indicates, but there is even no cause for suspicion of such a fact in 

 the apparently complete and continuous series of the " Sables infe- 

 rieurs" of the north of France. 



§ 3. Correlation of the several Divisions of the Lower Tertiaries. — 

 The Thanet Sands, and Landenien inferieur. 



To prove the foregoing position I will now state my reasons 

 for the correlation I propose to assign to each member of the Lower 

 Tertiaries of England, France, and Belgium, commencing with the 

 lowest, viz. the "Thanet Sands." This deposit ranges through that 

 part of the North of France which geologically forms a portion of 

 the Belgian Tertiary district. At the Artesian well of Calais it is, 

 as far as I could judge from the few specimens preserved in the 

 museum of that town, about 80 feet thick, or about the same as 

 on the opposite coast of Kent. Between Watten and St. Omer 

 the Lower Tertiary Sands crop out from beneath the London clay, 

 and the Thanet Sands reappear with characters closely analogous to 

 those which they present near Canterbury. During a hasty visit 

 to that district, I found in some semi-indurated beds numerous im- 

 pressions of shells, amongst which I recognised the Thracia ohlatay 

 the small Corbula common at Pegwell Bay, and traces of the same 

 Pholadomya, Pa^iopcBa, and Cy])rina. 



In the neighbourhood of Lille, M. A. Meugy * describes a series 

 of beds overlying the Chalk, and consisting of variable strata, from 1 5 

 to 105 feet thick, of dark grey or blackish sandy clay more or less 

 glauconiferous, fine sands, and semi-indurated calcareous marls, with 

 marine shells. These are precisely the characters the Thanet Sands 



* Essai de Geologie pratique sur la Flandre Fran^aise, Lille 1852, pp. 1 1 7-126. 



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