2i4 EOCENE PERIOD. [Ch, XVIII. 



several geologists, especially of M. Constant Prcvost *, have 

 led to great modifications in the theoretical views entertained 

 respecting the order in which the several groups were formed ; 

 and it now appears that the formations Nos. I, 2, and 3, of 

 the table of MM. Cuvier and Brongniart, instead of having 

 originated one after the other, are divisible into four nearly 

 contemporaneous groups. 



Superposition of different formations in the Paris basin. 

 A comparison of the two accompanying diagrams will enable 

 the reader to comprehend at a glance the different relations 

 which the several sets of strata bear to each other, according 

 to the original, as well as the more modern classification. 

 We shall now proceed to lay before the reader a brief sketch 

 of the several sets of strata referred to in the above systems. 



Immediately upon the chalk a layer of broken chalk flints, 

 often cemented into a breccia by siliceous sand, is very com- 

 monly found. These flints probably indicate the action of the 

 sea upon reefs of chalk when a portion of that rock had emerged 

 and before the regular tertiary beds were superimposed. To 

 this partial layer no reference is made in the annexed sections. 



Plastic clay and sand. Upon this flinty stratum, or, if it 

 be wanting, upon the chalk itself, rests frequently a deposit of 

 clay and lignite (No. 1 of the above tables). It is composed 

 of fresh-water shells and drift-wood, and was, at first., regarded 

 as a proof that the Paris basin had originally been filled with 

 fresh water. I3ut it has since been shown that this group is 

 not only of very partial extent, but is by no means restricted 

 to a fixed place in the series ; for it alternates with the marine 

 calcairc grossicr (No. 2 of the tables), and is repeated in the 

 very middle of that limestone at Veaugirard, Pagneux, and 

 other places, where the same Planorbes, Paluclinae, and Lim- 

 nei occur -j-. M. Desnoyers pointed out to me a section in the 

 suburbs of Paris, laid open in 1S2D, where a similar intercala- 

 tion was seen in a still higher part of the calcairc grossicr. 



* Build in cli-s Sci. di> hi Sue. Philmn.. May, 1S'25, p. 74. 



| Provost, Sur k-s Submersions Iterative*; &c. Mom. do la Sue. d'llist. Nut. 

 do Paris, tuine iv. p. 71. 



