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 widely different mineralogical composition, we may reasonably
suspect that the process of deposition has been more or less interrupted.
Nor will the closest inspection of a formation give us any idea of the
length of time which its deposition may have consumed. Many instances
could be given of beds, only a few feet in thickness, representing
formations which are 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 even 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 not have been suspected, had not the trees been preserved:
thus Sir C.
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