244 
EOCENE PERIOD. 
[Ch. XVIII. 
several geologists, especially of M. Constant Prevost *, 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. 1, 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. But 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 
calcaire grassier (No. 2 of the tables), and is repeated in the 
very middle of that limestone at Veaugirard, Bagneux, and 
other places, where the same Planorbes, Paludinae, and Lim- 
nei occur f . M. Desnoyers pointed out to me a section in the 
suburbs of Paris, laid open in 1829, where a similar intercala- 
tion was seen in a still higher part of the calcaire grassier. 
* Bulletin des Sci. de la Soc. Philom., May, 1825, p. 74. 
f Prevost, Sur Ies Submersions Iteratives, &c. Mem. de la Soc. d'Hist. Nat, 
de Paris, tome iv. p. 74. 
