30 Baron A. Humboldt on Petrifactions , or 
As in different countries the formations are not equally deve- 
loped, and shreds of formations may be taken for complete and 
entire ones, those which are destitute of shells in one region 
may present them in another. This consideration is important in 
obviating the pretty general tendency of multiplying formations 
in too great a degree ; for when, in a particular point of the 
globe, a formation (sandstone for example) abounds in its lower 
part with petrifactions, while its upper part is entirely destitute 
of them, this absence of petrifactions or fossils does not of itself 
justify the splitting of the deposit into two distinct formations. 
In the geological description of the neighbourhood of Paris, M. 
Brongniart has very properly united the millstones without 
shells, with those which are in a manner studded with fresh- 
water shells. 
We have seen that a formation may contain in different 
strata, petrifactions specifically different, but that most commonly 
some species of the lower stratum mingle with the great mass of 
heterogeneous species which occur united in the superimposed 
stratum. When this difference relates to genera, of which some 
are pelagic and others fresh-water shells, the problem of the 
unity or ind’ visibility of a formation becomes still more puzzling. 
Two cases must first be distinguished, that in which some flu vi- 
able shells occur, mixed with a great mass of marine shells, and 
that where marine and fluviatile shells may alternate bed by 
bed. Messrs Gilet de Laumont and Beudant, have made interest- 
ing observations regarding the admixture of marine and fresh- 
water productions, in one and the same bed. M. Beudant has 
proved, by ingenious experiments, how many of the fluviatile 
mollusca gradually habituate themselves to live in a fluid which 
has all the saltness of the ocean. The same geologist has, to- 
gether with M. Marcel de Serres, examined certain species of 
paludinse, which, preferring brackish waters, are found near our 
coasts, sometimes with pelagic shells, sometimes with fluviatile 
ones. (Journ. de Phys . t. lxxxiii. p. 137 ; t. lxxxviii. p. 211 ; 
Brongniart, Geogr. Min. p. 27, 54, 89.) To these curious facts 
are joined others, which I have published in the account of my 
Journey to the Equatorial Regions , (v. i. p. 535, and v. ii. 
p. 606), and which seem to explain what has formerly taken 
place upon the globe, according to what we still see taking place 
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