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Show 258 IMPERFECTION OF THE [CHAP. IX. the inordinately great chanO'e of climate, on the prodigious lapse of time, all incfuded within this sarr;e glacial period. Yet it Inay be doubted whether In any quarter of the world, sedimentary depo~its, i1!cl.uding fossil remains, have gone on accumulating within the same area dunng 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 marine animals can flourish~ for we know what vast geographical changes occurred in other parts of America during this space of tilne. When such beds as were 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 at first appear and disapp.ear 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 of the embedded fossils had been less than that of the glacial period, instead of having been really far greater, that is 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 have gone on accumulating for a very long period, in order to have given sufficient time for the slow process of variation; hence the deposit will generally have to be a 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 thick fossiliferous formation can only be accumulated during a period of subsidence ; and to keep the depth approximately the same, whi~h 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 movement 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 fact, this nearly exact balancin~ between the supply of sediment and the amount of subsidence is prob- CHAP. IX.] GEOLOGICAL RECORD. 259 ably a rare continO'ency. £ . •t h b more than one pafooontoloO'~~t It as een o.bserved by are usually barren of organTc ;.e~a~ very thiCk deposits upper or lower limits. ains, except near their It would seem that e h . ~hole pile of formations i~can separat~ formation, like the Intermittent in its accumulati~n~ou~£' has gene:}rally ?een often the case, a formation co en we see, a;s Is so mineralogical composition mposed of beds of different that the process of depositio:l m~y reasona?ly suspect as a change in the currents of thas een ~uch Interrupted, iment of a different nature will g e sea lln h a supply of sedgeographical changes re uirin enera y .ave been due to the closest inspection of aqform ~· muc.h time .. Nor will time which its deposition h a Ion give any Idea of the stances could he given of bed as consumed. .1\!any inrepresenting formations else~hlly a {hw feet in thickness, thi~kness, and which ~ust h ere ~usands of feet in period for their accu 1 · ave required an enormous fact would have Qus :~ ation; yet no one ignorant of this ed by the thinn:r fErm t~ the v;t lapse of time representof the lower beds of : f~I~ t. anh ca:ses could be given denuded, submerged and tl a IO~ aving been upraised, beds of the san1e for~ation lrfa I e-covere~ by the u~per yet easily overlooked inter~al hcts, showing 'Yh~t Wide, n1ulation In othe ' s ave occurred In Its accu- . r cases ·we h th 1 · in great fossilised trees still tved. e p ai?est evidence grew of man 1 · ' san Ing upright as they duridg the pi~c~s~gof~:r::i~· of tih~ and changes of level have been suspected haS ~otl; w ICh would never even been preserved: th~s Me~~. e trees chanced to have carl;JOniferous beds 1400 feet sth ~~el~ aNnd Dawso~ fou?d anCient root-bearin strat IC In ova Scotia, With than. sixty-eight di~erent ale~~!s abHe the other, at no less speCies occur at the bottom m. ddl ence, when the same tion, the probabilit . ' I e, and top of a formasame spot durin tif"Is, that t~ey have not lived on the disappeared andg e whol~ penod of deposition, but have the same geologi~!f~~~~d.' ~er~hp~ ·Tany times! during to undergo a considerable o t af I s~ch s~eCies were amoun o modrfication durin ao |
