nearly
....... 1866 1869 1872 | have 1859 1860 1861 |
counterbalance counterbalance 1866 1869 1872 | counterbalanced 1859 1860 1861 |
the amount of subsidence. But this same movement of subsidence will
....... 1869 1872 | often 1859 1860 1861 1866 |
tend to
submerge submerge 1866 1869 1872 | sink 1859 1860 1861 |
the area whence the sediment is derived, and thus diminish the
supply supply 1859 1860 1861 1866 1869 | supply, 1872 |
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. |
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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 different 1859 1860 1861 1866 1869 | widely different 1872 |
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. 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 |
Nor will the closest inspection of a formation give
any any 1859 1860 1861 1866 1869 | us any 1872 |
idea of the
time time 1859 1860 1861 1866 1869 |
length of time 1872 |
which its deposition
has has 1859 1860 1861 1866 1869 | may have 1872 |
consumed. Many instances could be given of beds only a few feet in thickness, representing formations,
elsewhere elsewhere 1859 1860 1861 1866 1869 |
which are elsewhere 1872 |
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 suspected 1859 1860 1861 1866 1869 | even suspected 1872 |
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
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