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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. 
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