250 
EOCENE PERIOD, 
[Ch. XVIII. 
Gyrogonites, or fossil seed-vessels of charse, are found abun- 
dantly in these strata, and all the animal and vegetable remains 
agree well with the hypothesis, that after the gulf or estuary 
had been silted up with the sand of the upper marine forma- 
tion, a great number of marshes and shallow lakes existed, like 
those which frequently overspread the newest parts of a delta. 
These lakes were fed by rivers or springs which contained, in 
chemical solution or mechanical suspension, such kinds of sedi- 
ment as Ave have already seen to have been deposited in the 
lakes of Central France during the Eocene period. 
The Parisian groups all Eocene. — Having now given a rapid 
sketch of the different groups of the Paris basin, we may 
observe generally that they all belong to the Eocene epoch, 
although the entire series must doubtless have required an 
immense lapse of ages for its accumulation. The shells of the 
different fresh-water groups, constituting at once some of the 
lowest and uppermost members of the series, are nearly all 
referable to the same species, and the discordance between the 
marine testacea of the calcaire grossier and the upper marine 
sands is very inconsiderable. 
A curious observation has been made by M. Deshayes, in 
reference to the changes which one species, the Cardium poru- 
losum, has undergone during the long period of its existence in 
the Paris basin. Different varieties of this cardium are cha- 
racteristic of different strata. In the oldest sand of the Sois- 
sonnais (a marine formation underlying the regular beds of the 
calcaire grossier),, this shell acquires but a small volume, and 
has many peculiarities which disappear in the lowest beds 
of the calcaire grossier. In these the shell attains its full size, 
and many peculiarities of form, which are again modified in the 
uppermost beds of the calcaire grossier, and these last cha- 
racters are preserved throughout the whole of the ' upper 
marine ' series *. 
Microscopic shells. — In some parts of the calcaire grossier 
microscopic shells are very abundant, and of distinct species 
* Coquilles characterise des Terrains, 1831, 
