When once, namely, a
generalisation has been established that certain fossils occur
in strata of a certain age, palaeontologists are apt to infer
that _all_ beds containing similar fossils must be of the same
age. There is a presumption, of course, that this inference would
be correct; but it is not a conclusion resting upon absolute
necessity, and there might be physical evidence to disprove it.
Fifthly, the physical geologist may lead the palaeontologist astray
by asserting that the physical evidence as to the age and position
of a given group of beds is clear and unequivocal, when such
evidence may be, in reality, very slight and doubtful. 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.
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