|
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 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
have been
had not the trees
→chanced to have
been preserved:
Lyell and
found carboniferous beds 1400 feet thick in Nova Scotia, with ancient root-bearing strata, one above the
at no less than sixty-eight different levels. Hence, when the same species
at the bottom, middle, and top of a formation, the probability is that
not lived on the same spot during the whole period of deposition, but
disappeared and reappeared, perhaps many times, during the same geological period.
→So that if such species
were to undergo a considerable amount of modification during
→any
one geological
a section would not
include all the fine intermediate gradations which must on
theory have
but abrupt, though perhaps
slight, changes of form. |
