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.
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