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.