these beds,
would would 1866 1869 1872 | might 1859 1860 1861 |
be tempted to conclude that the average duration of life of the embedded fossils had been less than that of the glacial period, instead of having been really far greater, that
is, is, 1861 1866 1869 1872 | is 1859 1860 |
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
must must 1859 1860 1861 1872 | will 1866 1869 |
have
gone on continuously accumulating during a gone on continuously accumulating during a 1872 |
gone on accumulating for a very 1859 1860 1861 |
to go on continuously accumulating during a very 1866 1869 |
long period,
sufficient sufficient 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 |
for the slow process of
modification; modification; 1869 1872 | variation; 1859 1860 1861 1866 |
hence the deposit
must must 1872 |
will generally have to 1859 1860 1861 |
will have to 1866 1869 |
be a very thick one; and the species
undergoing undergoing 1859 1860 1861 1869 1872 | under- going 1866 |
change must have lived in change must have lived in 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 |
the same
district district 1866 1869 1872 | area 1859 1860 1861 |
throughout
the the 1872 | this 1859 1860 1861 1866 1869 |
whole time. But we have seen that a thick
formation, formation, 1866 1869 1872 |
fossiliferous formation 1859 1860 |
formation, fossiliferous throughout its thickness, 1861 |
fossiliferous throughout its entire thickness, can accumulate only fossiliferous throughout its entire thickness, can accumulate only 1869 1872 |
can only be accumulated 1859 1860 1861 |
fossiliferous throughout its thickness, can accumulate only 1866 |
during a period of subsidence; and to keep the depth approximately the same, which is necessary
that that 1861 1866 1869 1872 |
in order to enable 1859 1860 |
the same
marine species may marine species may 1866 1869 1872 |
species to 1859 1860 |
species may 1861 |
live on the same space, the supply of sediment must 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, 1872 | supply 1859 1860 1861 1866 1869 |
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 widely different 1872 | different 1859 1860 1861 1866 1869 |
mineralogical composition, we may reasonably suspect that the process of deposition has been
more or less interrupted. more or less interrupted. 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 |
Nor will the closest inspection of a formation give
us any us any 1872 | any 1859 1860 1861 1866 1869 |
idea of the
length of time length of time 1872 |
time 1859 1860 1861 1866 1869 |
which its deposition
may have may have 1872 | has 1859 1860 1861 1866 1869 |
consumed. Many instances could be given of beds only a few feet in thickness, representing formations,
which are elsewhere which are elsewhere 1872 |
elsewhere 1859 1860 1861 1866 1869 |
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 even suspected 1872 | suspected 1859 1860 1861 1866 1869 |
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 not 1872 | never even 1859 1860 1861 1866 1869 |
have been
|