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. So far as our present
observation goes, none of the undoubted Graptolites have ever been
discovered in rocks later than those known upon other grounds
to be Silurian; but it is possible that they might at any time be
detected in younger deposits. Similarly, the species and genera
which we now regard as characteristic of the Lower Silurian, may
at some future time be found to have survived into the Upper
Silurian period. We should not forget, therefore, in determining
the age of strata by palaeontological evidence, that we are always
reasoning upon generalisations which are the result of experience
alone, and which are liable to be vitiated by further and additional
discoveries.

When the palaeontological evidence as to the age of any given
set of strata is corroborated by the physical evidence, our
conclusions may be regarded as almost certain; but there are
certain limitations and fallacies in the palaeontological method
of inquiry which deserve a passing mention. In the first place,
fossils are not always present in the stratified rocks; many
aqueous rocks are unfossiliferous, through a thickness of hundreds
or even thousands of feet of little-altered sediments; and even
amongst beds which do contain fossils, we often meet with strata
of many feet or yards in thickness which are wholly destitute
of any traces of fossils. There are, therefore, to begin with,
many cases in which there is no palaeontological evidence extant
or available as to the age of a given group of strata. In the
second place, palaeontological observers