38(J Proceedings of Philosophical Societies. , [Nov.. 



Article XII. 



Proceedings of Philosophical Societies. 



IMPERIAL INSTITUTE OP FRANCE. 



Account of the Labours of the Class of Mathematical and Physical 

 Sciences of the Imperial Institute of France during the Year 1813. 



{Continued from p. 311.) 



Mineralogy and Geology. 



The method of making correct observations becomes every day 

 more prevalent in geology, and we every day acquire more correct 

 notions respecting the strata of which every country is composed, 

 respecting the general laws of their superposition, and of the 

 organized bodies the remains of which these contain. 



The rocky beds containing only fresh-water shells, so great an ex- 

 tent of which was discovered in the environs of Paris by MM. Cuvier 

 and Brogniart, and which MM. Brogniart, Omalius de Halloy, M. 

 de Serres, Daudebart de Ferussac, &c. have found in a great number 

 of other countries, have particularly excited attention, and have in- 

 duced naturalists to make researches in order to distinguish fresh- 

 water shells from those of brackish and of sea water. MM. de 

 Ferussac and M. de Serres have each given a memoir on this subject. 

 The species alone, says the first of these gentlemen, can be brought 

 as evidence, and not the genera ; for most of the genera have both 

 marine and fresh-water species. It is not even useless to study the 

 varieties ; for the same species, according to the observations of the 

 same author, sometimes changes its form so much as not to be dis- 

 tinguished by one who has not attended to its different transitions ; 

 and the difficulty increases when we have fossil shells to determine, 

 when the epidermis, the hairs, and all the other characters of little 

 solidity, have disappeared. 



There are species, especially among the opercula, that are 

 capable of living in fresh-water, and which therefore are found 

 most abundantly at the mouths of rivers ; and we observe , among 

 the fossils traces of this habit ; for our fresh-water banks in certain 

 places contain a species of potomide^ a genus of shells usually 

 found at the mouths of rivers. 



M. Marcel de Serres visited on purpose the salt lakes on the coast 

 of the Mediterranean, to examine the shells that are found in them. 

 He observed pethidines very similar to those which form large banks 

 in the neighbourhood of Mcntz, along with which occur various 

 sea-shells. A geologist who had confounded these paludines with 

 the bulimias of our fresh-water beds, had concluded that these last 

 are sea-shells, as well as the others. But M. de Serres corrects this 



