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 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.