AND FRESH-WATER FORMATIONS. 327 



limestone, are therefore the fundamental strata of 

 these deposits when marine ; and when fresh water 

 strata succeed, they can be but modifications of the same 

 substances, while the coal series proves that all these 

 rocks are formed under such water, as does the tra- 

 vertino of Italy. The sources of the organic fossils 

 are no less obvious- 



But I must not pass from these, without inquiring 

 into their value in determining the marine or other 

 nature of these strata, This is especially necessary; 

 as the theory, and the mistakes of fact, together, have 

 been among the chief sources of erroneous judgment in 

 these cases, and will remain so as long as this engross- 

 ing pursuit shall occupy all the attention of geolo- 

 gists, and this hypothesis shall continue to rule. If 

 to mistake respecting a fish has been sufficent to con- 

 found the case of Oeningen, it is easy to see what 

 more may have happened and may happen again; 

 not only in such instances, but in the judgments re- 

 specting alternating deposits. 



I do not give catalogues of species and genera. 

 These concern zoology, not geological science: it is 

 a pursuit even more separated from the latter than 

 mineralogy ; and a geological system does not encum- 

 ber itself with catalogues of minerals. I shall only 

 therefore name, among the living genera of fresh 

 waters, Lymneus, Planorbis, Physa, Paludina, Ampul- 

 laria, Cerithium, Melanopsis, Melania, Nerita, Cyclas 

 and Unio. Of these, Lymneus, Planorbis, Physa, 

 Paludina, Cerithium., Melanopsis, Melania, and Ne- 

 rita, are found in the fossil state; and Paludina, 

 Ampullaria, Cerithium, Melania and Nerita are com- 

 mon to fresh and salt water. Of the shells called ex- 

 clusively marine, Modiolus, Mytilus, and Corbula, 

 live in fresh water; and different species of Anodon, 



