AUSTEN — EXTENSION OF THE COAL-MEASURES. 67 



land Oolite there, and which swarm with an obscure bivalve shell, 

 belong to the Purbeck series, so that the Umits of the lacustrine area 

 are defined in this direction. 



The occurrence of the Wealden Unio {U. Martini) through a con- 

 siderable vertical thickness of the Lower Cretaceous group of the 

 Department of the Haute-Marne points out the relative position of 

 the land at that time; and earHer still there were the Purbeck^ 

 Wealden conditions, indicated by the fluvio-marine formation de- 

 scribed by M. Cornuel*, and which are a continuation of those to be 

 observed in the Boulonnais. Independently of this agreement 

 between two localities somewbat distant, there is also this, that the 

 deposits of iron-ore, which represent subaerial conditions, exactly 

 agree as to position, so that it is on such considerations that the line 

 of limitation of the Neocomian group has been drawn from the Bou- 

 lonnais in a S.E. direction. 



Some sections near Beauvais, which were particularly examined 

 in 1852 by the Members of the Geological Society above named 

 (p. 66), show an alternation of marine Neocomian with Wealden 

 conditions, such as occurs at Punfieldf . In this way the area of 

 Wealden conditions becomes defined ; being bounded on one side by 

 the old ridge of the axis of Artois, and its extension to the S.E. ; on 

 the opposite side by a land-surface extending from our own western 

 districts towards and across France (Seine Inferieure) in aline parallel 

 with the Pays de Bray. Its northern limit would a priori have 

 been defined by the extension of the old axis of Artois ; and, if we 

 may take the northern chalk-escarpment of our Wealden denudation 

 to represent the N. limits of that axis, then the Wealden deposits 

 cannot be expected much beyond that line : or the Wealden lake was 

 defined on the N. by the extension of that old land which, as we have 

 shown, existed from early times to the N. of Belgium and Holland. 

 It was the same tract which in part supported the Secondary lacus- 

 trine deposits of Hanover. 



The place and form of the Wealden Valley become defined, as 

 does also the geological structure of the country around it ; and it is 

 thus that a peculiarity in the composition of its beds is satisfactorily 

 explained, — all the Wealden sandstones and conglomerates point to 

 their having been derived from the spoil of crystalline and meta- 

 morphic palseozoic strata ; and of this character would be all the 

 materials carried down by the streams from the old lands to the N., 

 as well as from those of the Channel area. 



The Wealden depression opened seawards on the S,W. 



The foregoing observations have reference to the probable abrupt 

 termination of the Wealden group on the N. Such a supposition 

 may perhaps appear unreasonable to those who consider only the 

 great dimensions of this series of depositions ; but such, however, is 

 the character of all the old lacustrine and many modern estuarine 

 formations with which we are acquainted : should time suffice for 



* Mem. de la Soc. Geol. de France, torn. iv. pp. 229-290. 

 t Topographic Geognostique du Dep. de I'Oise, par L. Graves; Lyell, Quart, 

 Joum. Geol. Soc. vol. vii. p. lix, 



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