and studies
the evidence of the fossils embedded in the rocks. Not only is
it thus much easier to determine the order of succession of the
strata in any given region, but it becomes now for the first time
possible to compare, with certainty and precision, the order of
succession in one region with that which exists in other regions
far distant. The value of fossils as tests of the relative ages
of the sedimentary rocks depends on the fact that they are not
indefinitely or promiscuously scattered through the crust of the
earth,--as it is conceivable that they might be. On the contrary,
the first and most firmly established law of Palaeontology is, that
_particular kinds of fossils are confined to particular rocks_,
and _particular groups of fossils are confined to particular
groups of rocks_. Fossils, then, are distinctive of the rocks in
which they are found--much more distinctive, in fact, than the
mere mineral character of the rock can be, for _that_ commonly
changes as a formation is traced from one region to another,
whilst the fossils remain unaltered. It would therefore be quite
possible for the palaeontologist, by an appeal to the fossils
alone, to arrange the series of sedimentary deposits into a pile
of strata having a certain definite order. Not only would this
be possible, but it would be found--if sufficient knowledge had
been brought to bear on both sides--that the palaeontological
arrangement of the strata would coincide in its details with the
stratigraphical or physical arrangement.

Happily for science, there is no such division between the
palaeontologist and the physical geologist as here supposed; but
by the combined researches of the two, it has been found possible
to divide the entire series of stratified deposits into a number
of definite _rock-groups_ or _formations_, which have a recognised
order of succession, and each of which is characterised by possessing
an assemblage of organic remains which do not occur in association
in any other formation. 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