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 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.