x INTRODUCTION. 
to present the forms and all the characters of alluvium; that is to 
say, a mixture of all sorts of heterogeneous matters, more fragments 
than entire bodies, coarse sands, unequal and irregular strata. 
We find nothing of this kind in the Fresh-Water Formations; the 
limestone is almost pure, every thing is in its place and perfectly 
entire, the most delicate shells present themselves in complete 
preservation; indeed, fragments of them are scarcely to be found 
in this formation, whilst, in the marine, fragments are met with in 
large quantities. We must, therefore, suppose a great tranquillity 
in the waters wherein these shells have lived. They are deposited 
in beds, often very thin and perfectly horizontal, like the masses 
of limestone and the zones of silex which they contain; the silex 
is in beds, frequently continuous, and never in rolled pieces. The 
disengagements of gas, indicated by the tubulures, have almost 
always been made vertically, and have often proceeded from the 
same plane, which is a further proof of the tranquillity of the liquid 
and the homogeneity of the Formation. Lastly, the immense extent 
of these deposits, which occupy spaces of more than 1200 square 
leagues, and which in so great a surface do not present any trace 
of disorder, renders this hypothesis perfectly admissible (1). 
The extent of these masses of Fresh- Water Formation ought not 
to astonish us; we know of others at least as vast in North America. 
If the lakes Superior, Michigan, Huron, Erie, and Ontario de- 
posited stony layers on their beds and became dry, they would 
leave Fresh-Water Formations more extensive than any of those 
just described (2). ; 
Among the different stages of alteration in which we find fossil 
shells, the most frequent is that in which only the animal part has 
(1) M. Brongniart remarks that marine and fresh-water shells have in no 
instance been found mixed together, except in the quarries of Grés at Beauchamp 
near Pierrelaie, where the cause is local ; for the fresh-water limestone, which 
forms the surface of the soil, reposes immediately on the marine sand which 
forms the bed or fond ; the mixture of the two, therefore, in this point of contact 
is natural. M. Defrance has since adduced and explained another instance ; 
see note on the Genus Helix. At Montmartre the Calcaire Marin forms a kind 
of mamelon or small hummock, and the gypsum in depositing itself on these 
beds, which are porous and friable, has enveloped the marine shells which 
they contain. Indeed, there is no proof that gypsum may not be a salt-water 
deposit; the upper marine formation contains small beds of gypsum at inter- 
vals, and the oysters are often covered with chrystals of selenite. There are 
only 3 fresh-water genera of bivalve shells, and it is remarkable that no species 
of either has been found in the Fresh Water Formation. 
(2) Fresh-water deposits must not be determined by the presence of doubt- 
ful shells, but, like that of the environs of Paris, on the constant presence of a 
“considerable majority of shells not found elsewhere, and on the constant 
absence of all marie bodies. See Pl. 4. 
