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AND FRESH-WATER FORMATIONS. .T27 limestone, are therefore the fundamental strata of these deposits when marine ; and when fresh water strata succeed, they can he 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 ot 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 oidy therefore name, among the living genera of fresh waters, Lymneus, Planorbis, Physa, Paludina, Ainpul- laria, Ccrithiuin, 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, Ainpullaria, 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,