Such an _assemblage of fossils_,
characteristic of any given formation, represents the _life_ of
the particular _period_ in which the formation was deposited.
In this way the past history of the earth becomes divided into a
series of successive _life-periods_, each of which corresponds
with the deposition of a particular _formation_ or group of strata.
Whilst particular _assemblages_ of organic forms characterise
particular _groups_ of rocks, it may be further said that, in
a general way, each subdivision of each formation has its own
peculiar fossils, by which it may be recognised by a skilled
worker in Palaeontology. Whenever, for instance, we meet with
examples of the fossils which are known as _Graptolites_, we may
be sure that we are dealing with _Silurian_ rocks (leaving out
of sight one or two forms doubtfully referred to this family).
We may, however, go much farther than this with perfect safety. If
the Graptolites belong to certain genera, we may be quite certain
that we are dealing with _Lower_ Silurian rocks. Furthermore, if
certain special forms are present, we may be even able to say to
what exact subdivision of the Lower Silurian series they belong.
As regards particular fossils, however, or even particular classes
of fossils, conclusions of this nature require to be accompanied
by a tacit but well-understood reservation.
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