| → formation, 1866 1869 1872 |
| fossiliferous formation 1859 1860 |
| formation, fossiliferous throughout its thickness, 1861 |
|
| → fossiliferous throughout its thickness, can accumulate only 1866 |
| can only be accumulated 1859 1860 1861 |
| fossiliferous throughout its entire thickness, can accumulate only 1869 1872 |
|
| → that 1861 1866 1869 1872 |
| in order to enable 1859 1860 |
|
| → marine species may 1866 1869 1872 |
| species to 1859 1860 |
| species may 1861 |
|
| → 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 |
|
| → time 1859 1860 1861 1866 1869 |
| length of time 1872 |
|
| → elsewhere 1859 1860 1861 1866 1869 |
| which are elsewhere 1872 |
|
|
the same
throughout
whole time. But we have seen that a thick
→formation,
→fossiliferous throughout its thickness, can accumulate only
during a period of subsidence; and to keep the depth approximately the same, which is necessary
→that
the same
→marine species may
live on the same space, the supply of sediment must nearly
the amount of subsidence. But this same movement of subsidence will
tend to
the area whence the sediment is derived, and thus diminish the
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
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
idea of the
→time
which its deposition
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
the vast lapse of time represented by the thinner formation. Many cases could be given of
|
