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tiary formations in America. Mr. Rogers's report recently laid before the British 

 Association for the Advancement of Science, gives a general outline of their ex- 

 tent, besides furnishing much valuable matter respecting them. No one, we pre- 

 sume, will dispute the talent and ability which he has displayed in the execution of 

 the task, but he has performed it under a conviction of the soundness of the new 

 principle in the arrangement of tertiary strata. He can infer, with precision, the 

 exact comparative age of a deposit by comparing its fossil shells with existing spe- 

 cies ! If we may hazard an opinion with reference to this subject, it would be 

 that the new principle, however beautiful in theory, or apparently simple in appli- 

 cation, as it at present stands, is as much a stumbling-block on the one hand as it 

 may be an assistance on the other. Mr. Conrad, it would appear, does not always 

 see his way so clearly as could be wished in making out his formations upon the 

 new system ; the per centages do not always tell up exactly as they ought. At 

 page 340 he observes, " I have rather too hastily supposed that the equivalent of 

 Mr. Lyell's miocene period occurred in this country ; but I am now convinced 

 that all above the eocene may more properly be termed older and newer pliocene. 

 There is no gradual transition from the older to the newer tertiary, but so vast 

 has been the change in the period of time which elapsed between them that a 

 single species of testacea has alone survived it ; besides, so many recent species of 

 the Atlantic coast of North America occur in every deposit of the tertiary above 

 the eocene, that although the amount varies considerably in different localities, 

 from fifteen to thirty per cent., yet I believe the discrepancy to have been caused 

 by different depths of water, or peculiarity of situation, not difference of time in 

 which the species existed. These remarks, however, do not apply to those depo- 

 sits which are composed almost exclusively of existing species ; they are certainly 

 entitled to the appellation of newer pliocene, and occur chiefly in Maryland, North 

 Carolina, and South Carolina." 



We cannot help wishing that Mr. Conrad had been a little more explicit in his 

 observation respecting the variation in the per centage of extinct fossil shells. As 

 the passage now stands it is involved in considerable obscurity. Every one must 

 be aware that in order to ascertain what proportion of fossil mollusca are identical 

 with existing forms in any one deposit, the comparison is made with species from 

 all depths and situations. The explanation given by Mr. Conrad is only applica- 

 ble upon the supposition that the recent types to which the fossil ones are referred 

 are exclusively littoral, or have all existed under similar physical conditions. 

 Then, indeed, we might reasonably infer that, in our examination of a fossiliferous 

 deposit, those localities would furnish us with the greatest number of recent spe- 

 cies in which the conditions which formerly existed most closely resembled those 

 from whence the living testacea had been obtained, and vice versa. 



If Mr. Conrad can bring forward evidence proving that deposits of the same 

 geological age exhibit a variation of fifteen per cent, in the number of extinct spe- 

 VOL. I. n 



