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 where 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, both in situ at Triel, and in 

 hand specimens in his cabinet. 



