But we have seen that a thick fossiliferous formation can
only be accumulated during a period of subsidence; and to keep the
depth approximately the same, which is necessary in order to enable
the same species to live on the same space, the supply of sediment
must nearly have counterbalanced the amount of subsidence. But this
same movement of subsidence will often tend to sink the area whence
the sediment is derived, and thus diminish the supply whilst the
downward movement continues. In fact, this nearly exact balancing
between the supply of sediment and the amount of subsidence is
probably a rare contingency; for it has been observed by more than one
palaeontologist, that very thick deposits are usually barren of
organic remains, except near their upper or lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of different mineralogical composition, we may
reasonably suspect that the process of deposition has been much
interrupted, as a change in the currents of the sea and a supply of
sediment of a different nature will generally have been due to
geographical changes requiring much time. Nor will the closest
inspection of a formation give any idea of the time which its
deposition has consumed. Many instances could be given of beds only a
few feet in thickness, representing formations, elsewhere thousands of
feet in thickness, and which must have required an enormous period for
their accumulation; yet no one ignorant of this fact would have
suspected the vast lapse of time represented by the thinner formation.
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