222 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



took place at the commencement of the London Clay period, and 

 whereby a large body of water, suddenly displaced and passing over 

 the new-formed sea-bed, swept the banks of pebbles formed on the 

 sea-shore and spread over the littoral zone during the Woolwich and 

 Reading period, leaving them in some places merely as a thin layer, 

 and at other places drifting them into thick heaps. 



There are apparently in the French series some analogous condi- 

 tions. There are many large accumulations of pure shingle ; but no- 

 where have these been traced under any of the regular strata, as we 

 can trace the shingle bed of Plumstead under the London Clay of 

 Shooter's Hill. In those places where the fossiliferous sands over- 

 lying the Lignites are pebbly, the fossils are evidently such as 

 belong to the Bracheux group, and I have not recognized in them 

 any of the more ordinary and exclusive London Clay forms. Where 

 the pebbly sands and sandstones are overlaid by other strata, — the 

 '* sables divers" of M. d'Archiac, and *' Glauconie moyenne" of 

 M. Graves, — another series of fossils commences ; amongst these are 

 the Pectunculns depressus, Ostrea flabellula, Crassatella lamellosa, 

 Cassidaria carinata, and other shells not found in the lower sands, 

 but belonging to the Calcaire grossier group. (See Table.) 



Here then we have a divergence in the London and Paris series. 

 I doubt whether any of the beds just described are the exact equi- 

 valents of the Basement bed of the London Clay ; whether any of 

 the great isolated masses of pebbles on the confines of the Paris 

 district were then, as in England, swept into their present place 

 from their previous position in the underlying sands and clays. The 

 general appearances are certainly in favour of the existence of like 

 conditions, at that period, in the two countries ; but they are merely 

 such physical conditions as might have resulted at any period by the 

 movement of a body of water over the loose materials of the lower 

 sands. In England, this followed close upon the former period of 

 the Woolwich and Reading series ; but it is a question whether in 

 France the " Lignite and Argile Plastique " period was not followed 

 by a period of dry land, and whether the partial destruction and 

 reconstruction of the surface of some of the beds of that period did 

 not take place later, viz. when the land was again submerged, and 

 when consequently some portion of the pebbles and shingle derived 

 from the lower beds were spread out at the base of the deposits of 

 another period, — the one commencing with the Lits Coquilliers and 

 Calcaire grossier. The probability of this position will be better 

 understood after an examination of the overlying beds. 



§ 7. The relation of the lower members of the French series to the 

 Sables Divers and Lits Coquilliers {D'Archiac), or the Glauconie 

 Moyenne (Graves). 



In England the shingle beds are immediately succeeded by the 

 London Clay, and in the northern part of the Paris basin by M. 

 d'Archiac's fourth division, "Sables divers," which consists of light- 

 yellow and greenish fine quartzose sands, attaining in places a thick- 

 ness of 140 feet, but only occasionally containing a very few fossils. 



