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1859
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gone on accumulating for a very 1859 1860 1861
to go on continuously accumulating during a very 1866 1869
gone on continuously accumulating during a 1872

in order to have given sufficient time 1859 1860
so that there may have been time sufficient 1861
so that there may be time sufficient 1866 1869
sufficient 1872

will generally have to 1859 1860 1861
will have to 1866 1869
must 1872

modification will have had to live on 1859 1860 1861
modification will have to live in 1866
change will have to live in 1869
change must have lived in 1872

fossiliferous formation 1859 1860
formation, fossiliferous throughout its thickness, 1861
formation, 1866 1869 1872

can only be accumulated 1859 1860 1861
fossiliferous throughout its thickness, can accumulate only 1866
fossiliferous throughout its entire thickness, can accumulate only 1869 1872

in order to enable 1859 1860
that 1861 1866 1869 1872

species to 1859 1860
species may 1861
marine species may 1866 1869 1872

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. 1859 1860 1861 1866 1869
more or less interrupted. 1872

of the embedded fossils had been less than that of the glacial period, instead of having been really far greater, that
is,
is
extending from before the glacial epoch to the present day.
In order to get a perfect gradation between two forms in the upper and lower parts of the same formation, the deposit
will
must
have gone on accumulating for a very long period, in order to have given sufficient time for the slow process of
modification;
variation;
hence the deposit will generally have to be a very thick one; and the species
under- going
undergoing
modification will have had to live on the same
district
area
throughout
the
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
have
counterbalance
counterbalanced
the amount of subsidence. But this same movement of subsidence will
often
often
tend to
submerge
sink
the area whence the sediment is derived, and thus diminish the
supply,
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 palæontologist, 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
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.