94 REPORT — 1872. 



may safely bo placed to tue nor tli of the Boulonnais denudation ; for the Wealden 

 depositions proper hardly rise to the level of the Palseozoic roclcs of Marquise. The 

 great fissures and pot-lioles in the limestones there, which have been produced 

 under subaerial conditions, and filled with sand, mould, and much vegetable matter, 

 had been produced antecedently to the deposition of the Gault over that area. 



The Wealden beds of the Boulonnais were formed beneath the waters of the 

 same lake as our own. This freshwater area had an extension southwards ; thus 

 M. d'Archiac refer3;^the mottled clays beneath the iron-sands and sandstones at 

 Ilavi'e to the Wealden series of this country ; so that the limits of our lake in that 

 direction, or in the south, lay somewhere along the line of the English Channel. 



Sixty miles to the south of the Boulonnais is a district known as the Pays de 

 Bray, which is an elliptical valley of elevation and denudation, like our own 

 Wealden on a small scale, extending from Beauvais to Neufchatel, a distance of 

 forty-five miles. In this denudation the lowest beds exposed belong to the marine 

 Jurassic series (Portland Kiuimeridge). Next above the Portland stone is a 

 Wealden formation. "Les depots regardes comme fluviatiles sont les plus voisins 

 de I'etage Portlandien, et ferment le groupe iuferieur du terrain Neocomien " 

 (Graves, Oise, p. 55). The remains of the fishes, Cyrencv, Ci/prides, and ferns are 

 such as occur in our Wealden. 



Tlie thickness of tliis freshwater formation is inconsiderable compared with our 

 Wealden. The separation of the freshwater formation from the marine Portland 

 is well defined ; not so that betwixt the Wealden and Neocomian : here, as in the 

 Pimlield section, the freshwater and marine conditions seem to have alternated ; and 

 the manner in which this takes place suggests tlie siipposition that the influx of a 

 considerable body of fresh water from tlie land of tlie time took place not far 

 from this place. 



Neufchatel is seven t}^ miles south of Boulogne ; the Wealden beds, as we have 

 seen, indicate that the series extended southwards from Marquise ; and it is no un- 

 reasonable supposition that the deposits of the Pays de Bray were formed under 

 the waters of the same lake as were those of our o-wm Wealden. 



Such, then, Avere the dimensions of the Wealden lake, or sound. It extended 

 from parts of Buckingham, on the north, half across the English Channel on the 

 south, a breadth of 160 miles ; in the contrary direction it reached from Wiltshire 

 far into France, beyond Beauvais for 250 miles. 



In another part of France, Depart, de I'Aube, M. Comuel has described a fluvio- 

 lacustrine formation between the Jm'assic and Cretaceous formations at Vassy, 

 containing Ic/iumodon, several .species of Unio, and Planorhis. The lacustrine for- 

 mation at Cimey is in a corresponding geological position. 



In the Jura, Yillers, Forcine-le-bas, the Portland beds are followed by hard 

 bluish marls, calcareous marls, and gjiisum, the whole very like our Purbeck series. 

 These lacustrine formations are interesting, as they seem to show the existence of a 

 chain of lakes stretching across France into Switzerland for 2G0 miles, with a 

 general direction parallel to the axis of Artois, and thus connected as part of one 

 great lake-system with our Wealden. 



In France, Dep. des Deux Charentes, some 3oO miles due south of our Sussex 

 coast, there occurs a great freshwater formation in intermediate position between 

 the Portland Oolite and what were then the lowest beds of the Cretaceous scries. 

 Like our own AVealden, this also is exhibited over a surface from which the Creta- 

 ceous strata have been demulcd. This formation has engaged the attention of many 

 French geologists, more particularly of M. Coquand, who has determined its age 

 and purely lacustrine character, and who puts it as the equivalent of the Purbeck 

 beds of England ; in this he seems to be guided by the general likeness as to com- 

 position and the presence of Physu Bridowi, a well-known Purbeck species. 



The sequence of events at this place was as follows : — Subsequently to the for- 

 mation of the Portland Oolite the sea-bed became terrestrial surface ; and subse- 

 quently again to that a depression, extending from Chateauneuf, near Angouleme, 

 to beyond the Island of Oleron, became the site of a gTeat freshwater lake. From 

 St. Jean d'Angely to Chateauneuf is a distance of thirty-five miles ; and from 

 Chateauneuf to Oleron, S.E. to N.W., is upwards of 100 miles ; but then figures 

 do not give the full dimensions of this freshwater area, aa its deposits have 



