The principal cause
of this common phenomenon is what is known as "metamorphism"--that
is, the subjection of the rock to a sufficient amount of heat to
cause a rearrangement of its particles. When at all of a pronounced
character, the result of metamorphic action is invariably the
obliteration of any fossils which might have been originally
present in the rock. Metamorphism may affect rocks of any age,
though naturally more prevalent in the older rocks, and to this
cause must be set down an irreparable loss of much fossil evidence.
The most striking example which is to be found of this is the
great Laurentian series, which comprises some 30,000 feet of
highly-metamorphosed sediments, but which, with one not wholly
undisputed exception, has as yet yielded no remains of living
beings, though there is strong evidence of the former existence
in it of fossils.
Upon the whole, then, we cannot doubt that the earth's crust, so
far as yet deciphered by us, presents us with but a very imperfect
record of the past. Whether the known and admitted imperfections
of the geological and palaeontological records are sufficiently
serious to account satisfactorily for the deficiency of direct
evidence recognisable in some modern hypotheses, may be a matter
of individual opinion. There can, however, be little doubt that
they are sufficiently extensive to throw the balance of evidence
decisively in favour of some theory of _continuity_, as opposed
to any theory of intermittent and occasional action.
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