In this way, the observer may be readily led into wrong conclusions as to the nature of the organic remains--often obscure and fragmentary--which it is his business to examine, or he may be led erroneously to think that previous generalisations as to the age of certain kinds of fossils are premature and incorrect.
Lastly, there are cases in which, owing to the limited exposure of the beds, to their being merely of local development, or to other causes, the physical evidence as to the age of a given group of strata may be entirely uncertain and unreliable, and in which, therefore, the observer has to rely wholly upon the fossils which he may meet with.
In spite of the above limitations and fallacies, there can be no doubt as to the enormous value of palaeontology in enabling us to work out the historical succession of the sedimentary rocks.
It may even be said that in any case where there should appear to be a clear and decisive discordance between the physical and the palaeontological evidence as to the age of a given series of beds, it is the former that is to be distrusted rather than the latter.

The records of geological science contain not a few cases in which apparently clear physical evidence of superposition has been demonstrated to have been wrongly interpreted; but the evidence of palaeontology, when in any way sufficient, has rarely been upset by subsequent investigations.

Should we find strata containing plants of the Coal-measures apparently resting upon other strata with Ammonites and Belemnites, we may be sure that the physical evidence is delusive; and though the above is an extreme case, the presumption in all such instances is rather that the physical succession has been misunderstood or misconstrued, than that there has been a subversion of the recognised succession of life-forms.
We have seen, then, that as the collective result of observations made upon the superposition of rocks in different localities, from their mineral characters, and from their included fossils, geologists have been able to divide the entire stratified series into a number of different divisions or formations, each characterised by a _general_ uniformity of mineral composition, and by a special and peculiar _assemblage_ of organic forms.

Each of these primary groups is in turn divided into a series of smaller divisions, characterised and distinguished in the same way.

It is not pretended for a moment that all these primary rock-groups can anywhere be seen surmounting one another regularly.[8] There is no region upon the earth where all the stratified formations can be seen together; and, even when most of them occur in the same country, they can nowhere be seen all succeeding each other in their regular and uninterrupted succession.