time for the slow process of variation; hence the deposit will
generally have to be a very thick one; and the species undergoing
modification will have had to live on the same area throughout this
whole time. 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 pal ontologist,
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. Many cases could be given of the lower beds of a
formation having been upraised, denuded, submerged, and then re-covered
by the upper beds of the same formation, facts, showing what wide, yet
easily overlooked, intervals have occurred in its accumulation. In
other cases we have the plainest evidence in great fossilised trees,
still standing upright as they grew, of many long intervals of time and
changes of level during the process of deposition, which would never
even have been suspected, had not the trees chanced to have been
preserved: thus, Messrs. Lyell and Dawson found carboniferous beds 1400
feet thick in Nova Scotia, with ancient root-bearing strata, one above
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