CONCLUSIONS TO BE DRAWN FROM FOSSILS.
We have already seen that geologists have been led by the study
of fossils to the all-important generalisation that the vast
series of the Fossiliferous or Sedimentary Rocks may be divided
into a number of definite groups or "formations," each of which is
characterised by its organic remains. It may simply be repeated here
that these formations are not properly and strictly characterised
by the occurrence in them of any one particular fossil. It may be
that a formation contains some particular fossil or fossils not
occurring out of that formation, and that in this way an observer
may identify a given group with tolerable certainty. It very often
happens, indeed, that some particular stratum, or sub-group of a
series, contains peculiar fossils, by which its existence may
be determined in various localities. As before remarked, however,
the great formations are characterised properly by the association
of certain fossils, by the predominance of certain families or
orders, or by an _assemblage_ of fossil remains representing
the "life" of the period in which the formation was deposited.
Fossils, then, enable us to determine the _age_ of the deposits
in which they occur. Fossils further enable us to come to very
important conclusions as to the mode in which the fossiliferous
bed was deposited, and thus as to the condition of the particular
district or region occupied by the fossiliferous bed at the time
of the formation of the latter.
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