246 
EOCENE PERIOD. 
[Ch. -XVIII. 
part of the Paris basin, another contemporaneous deposit, of 
fresh-water origin, appears at the southern extremity. 
Calcaire siliceux. — This group (No. 3 of the foregoing 
tables) is a compact siliceous limestone, which resembles a 
precipitate from the waters of mineral springs. It is, for the 
most part, devoid of organic remains, but in some places it con- 
tains fresh-water and land species, and never any marine fossils. 
The siliceous limestone and the calcaire grossier occupy distinct 
parts of the basin, the one attaining its fullest development in 
those places wh-ere the other is of slight thickness. They also 
alternate with each other towards the centre of the basin, as at 
Sergy and Osny, and there are even points where the two rocks 
are so blended together, that portions of each may be seen in 
hand specimens. Thus in the same bed, at Triel, we have 
the compact fresh-water limestone, characterized by its Limnei, 
mingled with the coarse marine limestone through which the 
small multilocular shell, called milliolite, is dispersed in count- 
less numbers. These microscopic testacea are also accom- 
panied by Cerithia and other shells of the calcaire grossier. It 
is very extraordinary that, although in this instance both kinds 
of sediment must have been thrown down together on the same 
spot, each still contains its own peculiar organic remains*. 
These facts lead irresistibly to the conclusion, that while to 
the north, where the bay was probably open to the sea, a ma- 
rine limestone was formed, another deposit of fresh-water 
origin was introduced to the southward, or at the head of the 
bay. For it appears that during the Eocene period, as now, 
the ocean was to the north, and the continent, where the great 
lakes existed, to the south. From the latter region we may 
suppose a body of fresh water to have descended charged with 
carbonate of lime and silica, the water being perhaps in suf- 
ficient volume to convert the upper end of the bay into fresh 
water^ like some of the gulfs of the Baltic. 
Gypsum and marls. — The next group to be considered is 
* M. Prevost has pointed out this limestone to me, hoth in situ at Triel, and in 
hand specimens in his cahinet. 
