X 



INTRODUCTION 



lo 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 Gres 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 marine bodies. See PL i. 



