•tlO MAKUAI OF THE MOLLUSCA. I 



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It must be observed that the number and magnitude of the " Formations " ■] 

 was determined by accident in the first instance, and afterwards modified to 

 suit tile requirements of theory, and to make them more nearly equal in value.* 



According to MM. Agassiz and D'Orbigny, all, or nearly all the fossils of ' 

 each formation are peculiar; very i^vf species being supposed to havesm-yived 

 from one period to another. Sudden and entire chciuges ^f this kind only 

 take place when the nature of the deposit is completely altered, — as when 

 sauds or clays rest upon chalk ;— and in these instances there is usually evi- 

 dence (iu the form of beds of shingle, or a change of dip) that an interval \ 

 must have elapsed between the completion af the lower stratum and the com- i 

 luencement of the upper. 



We have seen that distinct faunas may be separated by narrow barriers in \ 

 existing seas ; and difi"erences almost as great may occur on the same coast- '| 

 line without the interposition of any barrier^ merely in passing from a sea-bed ] 

 of rock and weed to one of Sand or mud, or to a zone of different depth. It \ 

 would be unreasonable to expect the same fossils in a limestone as in a sand- 

 stone ; and even iu comparing similar strata we must consider the probability 

 of their having been formed at different depths, or in distinct zoological pro- .1 

 vinces. 



The most careful observations hitherto made, under the most favourable 

 circumstances, tend to show that all sudden alterations have beeu local, and 

 that the law of change over the whole globe, and through all time, has bten ., 

 gradual and uniform. The hypothesis of Sir C. Lyell— that species have been ',; 

 created, and have died out, 07ie by one, agi-ees far better with facts, than the j 

 doctrine of periodic and general extinctions and creations. 'I 



As regards the Zoological value of the " formations," we shall be within 1 

 the truth if we assume that those already established correspond in import- \ 

 ance with geographical provinces ; for at least half the species are peculiar, \ 

 the remainder being common to the previous or succeeding strata. This will 

 give to each Geological Period a length equal to three times the average 

 duration of the species of marine sheHs.f 



* The names of Formations are in great measure provisional, and open to criticism. 

 Some of them were given In- Brongniart and O. D'Halloy; others have been more' 

 recently applied by D'Orbigny, Sedgwick, Murchison, and Barrande; and some are 

 adopted from popular usage. Geogr aj.hical names, and those derived from charac- 

 teristic fossils have been found the best, but no complete scheme of zoolocjical nomen- 

 ctatuie has been framed. 



The epithet " Turonien " (25) is rejected, because it conveys the same meaning with 

 " Falunian" (29), or Middle Tertiary, the type of which Vras taken from Touraine. 



The term Icenian is proposed for the Pliocene strata, because their order of suc- 

 cession was iirst determined, by Mr. Charlesworth, in tlie eastern counties of England 

 the country of the Iceni.- 



+ The exact value of these periodi cannot be ascertained, but some notion of trieir 

 length may be obtained by considering that the deposits in the valley of the Mississippi 



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