CHAPTER V.
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.