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Show 294 IMPERFECTION OF THE CHAP. IX. only then first immigrated into tha~ area. It is well known, for instance, that several species appeared somewhat earlier in the palooozoic beds of North America than in those of Europe; time having apparently been required for their migration from the American to the European seas. In examining. the latest deposits of various quarters of the world, 1t has everywhere been noted, that some few still existing species are common in the deposit, but have become extinct in the immediately surrounding sea ; or, conversely, that some are now abundant in the neighbouring sea, but are rare or absent in this particular deposit. It is an excellent lesson to reflect on the ascertained amount of migration of the inhabitants of Europe during the Glacial period, which forms only a part of one whole geological period; and likewise to reflect on the great changes of level, on the inordinately great change of climate, on the prodigious lapse of time, all included within this same glacial period. Yet it may be doubted whether in any quarter of the world, sedimentary deposits, including fossil remains, have gone on accumulating within the same area during the whole of this period. It is not, for instance, probable that sediment was deposited during the whole of the glacial period near the mouth of the Mississippi, within that limit of depth at which rna~ rine animals can flourish ; for we know what vast geographical changes occurred in other parts of America during this space of time. When such beds as we~·e deposited in shallow water near the mouth of the Mississippi during some part of the glacial period shall have been upraised, organic remains will probably first appear and disappear at different levels, owing to the migration of species and to geographical changes. And in ~the distant future, a geologist examining these beds, mi~ht be tempted to conclude that the average duration of hfe CHAP. IX. GEOLOGICAL RECORD. 295 of the embedded fossils had been less than that of th glacia}- period, !nstead of having been really far greate:. that Is extending from before the glacial epoch to the present day. . In order to get a perfect gradation between two forms m th~ upper and lower parts of the same formation, the dep?sit ~ust have gone on accumulating for a very long period, 1n order to have given sufficient time for the slow process of variation ; hence the deposit will generally ~ave to .be ~very. thick one; and the species undergoing modification will have had to live on the same area throughout this whole time. But we have seen that a thi?k fossil~erous formation can only be accumulated dunng ~ penod 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 counterbalanced the amount of subsidence. But this same 1novement of subsidence will often tend to sink the area whence the sediment is derived, and thus diminish the supply whilst the downward movement continues. In fac~, 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 palreontologist, that very thick deposits are usually barren of organic remains, except near their upper or lower limits. It wo~d seem that each separate formation, like the whole. pile of formations in any country, has generally bee~ Intermittent in its accumulation. When we see, as I~ so often the case, a formation composed of beds of different mineralogical composition, we may reasonably s~pect that the process of deposition has been much mterrupted, as a change in the currents of the sea and a supply of sediment of a different nature will |
