218 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



depth of that series common physical characters, common mineral 

 structure, and common fossils, showing an intimate relation of con- 

 ditions throughout the whole period, I cannot but consider that in 

 France likewise, notwithstanding less blending of the several parts, 

 the lignites of the Soissonnais and fluviatile clays are, with the 

 mottled clays, subordinate to an equivalent but more largely deve- 

 loped mass of marine sands. Where the organisms of two groups of 

 strata are so different as they necessarily must be in these marine 

 sands and the overlying fluviatile clays, an apparent distinctiveness 

 is produced by the difference of conditions, the reality of which can 

 only be tested by a recurrence to like terms of comparison. 



In this country we have seen that the marine sands pass horizon- 

 tally into estuarine sands underlying the fluviatile clays ; that estuarine 

 and marine sands, with a group of shells identical with those descend- 

 ing into the clays and underlying sands, occur above these fluviatile 

 clays ; whilst the whole of these three subdivisions pass further 

 westward into one undistinguishable series of unfossiliferous mottled 

 clays and subordinate sands. Now, in examining closely the French 

 series, somewhat similar phsenomena are apparent. M. Graves has 

 noticed in several places the occurrence of sands with marine shells 

 over the lignites ojp the Soissonnais ; and so marked is this feature at 

 Varesne and St. Sauveur near Pont St. Maxence, that M. d'Archiac, 

 looking at these beds as the equivalent of the marine sands at Bra- 

 cheux, has on this evidence been inclined of late to place these latter 

 above the lignites, contrary to the view he first took of their super- 

 position, and which I believe to be the correct one * . The occurrence 

 of 9 to 10 feet of sand, with marine shells, such as Pectunculus tere- 

 bratularis, Ostrea Bellovacina, Nucula, Cardiuniy Venericardia, Ceri- 

 thiunii and also the Cyrena cuneiformis^ in a similar position, has 

 been noticed at several other places in the departments of the Somme, 

 Oise, and Aisnef . Although the fossils of these beds have not been 

 thoroughly examined, the notices given of them, and the few I have 

 seen, lead me to believe that they are of the same species as those 

 found in the Bracheux sands, and consequently that we have, in 

 these beds overlying the fluviatile clays, a repetition of marine con- 

 ditions such (but to a less extent) as prevailed before the intercalation 

 of the subordinate clays ; these latter appearing therefore, as in 

 England, in the light of a temporary and local accumulation of river 

 or lagune sediment spread over ground from which the sea had par- 

 tially retired, but which at a later epoch it again invaded. 



Wherever the fluviatile, estuarine, and marine conditions of this 

 division of the Lower Tertiaries (4, 5, & 6 (part?), D'Archiac) are 

 in full force, there the mottled clay (the true argile plasticpie of the 

 French geologists) ceases altogether or in greater part. In the 

 neighbourhood of Paris, the lignites and fluviatile beds form occa- 

 sional bands (" fausses glaises") overlying the '^ Plastic Clay." This 

 order of superposition — that the Plastic Clay forms the base of the 



* 1839, Bull. vol. X. p. 172. See also further on, p. 245, for section at Coivrel. 

 t See Top. Geog. du Dep. de I'Oise, p. 234 ; section of the Lagny pit, p. 211, 

 and mention of sandstone blocks at Louvetain, p. 224. 



