M. Brongniart on the Fresh-water Formations of Italy. 101 
summit of which Pomarance is situated ; it is nevertheless al- 
ways superior to the marine limestone, and to thejgypseous 
marles which are beneath, because all these formations have fol- 
lowed the same inclination, and seem to terminate here, since, 
on the other side, we find ourselves upon another formation, 
composed of micaceous compact limestone, covered with ophio- 
lite. 
I shall not prosecute farther my inquiries regarding the fresh- 
water formations. The numerous examples which I have ad- 
duced are sufficient for giving an idea of the great extent of 
a formation to which, until ten years ago, no attention had 
been paid ; and for presenting to the view of naturalists the re- 
markable analogy of their characters, in all these places, so re- 
mote from each other, and so different with regard to the na- 
ture of the subjacent rock. 
I have insisted upon the fresh-water formations which are at 
the present day in the act of forming, because they afford us 
means of appreciating the causes which may and must have pro- 
duced the deposits of ancient formation, and consequently of 
establishing the theory of those of the neighbourhood of Paris. 
The fresh-water formations of Rome, of Sienna, of Colie, and 
of Pomerance, are in a geological situation identically the same 
as those of Fontainebleau, of the Plaine de Trappe, of Mont- 
morency, &c. We cannot refuse to attribute to these calcareous 
waters issuing from the bosom of the earth, and from beneath 
the most ancient sedimentary formation, the construction of the 
fresh-water formations of Italy and Hungary. Results perfect- 
ly similar must be attributed to the same cause. We may 
therefore presume, that the fresh-water formations of the neigh- 
bourhood of Paris, are owing to an abundance of calciferous 
and siliciferous thermal springs, which are dried up, as those of 
Pomarance already are, and as those of Colie are about to be, 
whether their reservoir has been exhausted, or their canals ob- 
structed. 
It is true that we find, in the neighbourhood of Paris, fresh- 
water formations entirely siliceous, and that none of this kind 
occur in the parts of Italy which I have mentioned ; but I 
shall recall to mind, that those of Colie contain very distinct si- 
liceous parts ; and, besides, if the examination of this circum- 
stance would not lead me altogether beyond the natural limits 
