We should be misled in taking the thickness of that layer, whenever it
may be exposed to our view, as a record of time in the manner in which
we are now regarding this subject, as it would give us only an
imperfect and partial record: it would seem to represent too short a
period of time.
Fig.4.
Suppose, on the other hand, that the land (C D) had gone on rising
slowly and gradually--say an inch or two inches in the course of a
century,--what would be the practical effect of that movement? Why,
that the sediment A and B which has been already deposited, would
eventually be brought nearer to the shore-level, and again subjected to
the wear and tear of the sea; and directly the sea begins to act upon
it, it would of course soon cut up and carry it away, to a greater or
less extent, to be re-deposited further out.
Well, as there is, in all probability, not one single spot on the whole
surface of the earth, which has not been up and down in this way a
great many times, it follows that the thickness of the deposits formed
at any particular spot cannot be taken (even supposing we had at first
obtained correct data as to the rate at which they took place) as
affording reliable information as to the period of time occupied in its
deposit. So that you see it is absolutely necessary from these facts,
seeing that our record entirely consists of accumulations of mud,
superimposed one on the other; seeing in the next place that any
particular spots on which accumulations have occurred, have been
constantly moving up and down, and sometimes out of the reach of a
deposit, and at other times its own deposit broken up and carried away,
it follows that our record must be in the highest degree imperfect, and
we have hardly a trace left of thick deposits, or any definite
knowledge of the area that they occupied, in a great many cases.
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