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subsidence¡£¡¡Since¡¡publishing¡¡my¡¡views¡¡on¡¡this¡¡subject¡¡in¡¡1845£»¡¡I¡¡have¡¡watched¡¡the¡¡progress¡¡of¡¡Geology£»¡¡and¡¡have¡¡been¡¡surprised¡¡to¡¡note¡¡how¡¡author¡¡after¡¡author£»¡¡in¡¡treating¡¡of¡¡this¡¡or¡¡that¡¡great¡¡formation£»¡¡has¡¡come¡¡to¡¡the¡¡conclusion¡¡that¡¡it¡¡was¡¡accumulated¡¡during¡¡subsidence¡£¡¡I¡¡may¡¡add£»¡¡that¡¡the¡¡only¡¡ancient¡¡tertiary¡¡formation¡¡on¡¡the¡¡west¡¡coast¡¡of¡¡South¡¡America£»¡¡which¡¡has¡¡been¡¡bulky¡¡enough¡¡to¡¡resist¡¡such¡¡degradation¡¡as¡¡it¡¡has¡¡as¡¡yet¡¡suffered£»¡¡but¡¡which¡¡will¡¡hardly¡¡last¡¡to¡¡a¡¡distant¡¡geological¡¡age£»¡¡was¡¡certainly¡¡deposited¡¡during¡¡a¡¡downward¡¡oscillation¡¡of¡¡level£»¡¡and¡¡thus¡¡gained¡¡considerable¡¡thickness¡£¡¡
All¡¡geological¡¡facts¡¡tell¡¡us¡¡plainly¡¡that¡¡each¡¡area¡¡has¡¡undergone¡¡numerous¡¡slow¡¡oscillations¡¡of¡¡level£»¡¡and¡¡apparently¡¡these¡¡oscillations¡¡have¡¡affected¡¡wide¡¡spaces¡£¡¡Consequently¡¡formations¡¡rich¡¡in¡¡fossils¡¡and¡¡sufficiently¡¡thick¡¡and¡¡extensive¡¡to¡¡resist¡¡subsequent¡¡degradation£»¡¡may¡¡have¡¡been¡¡formed¡¡over¡¡wide¡¡spaces¡¡during¡¡periods¡¡of¡¡subsidence£»¡¡but¡¡only¡¡where¡¡the¡¡supply¡¡of¡¡sediment¡¡was¡¡sufficient¡¡to¡¡keep¡¡the¡¡sea¡¡shallow¡¡and¡¡to¡¡embed¡¡and¡¡preserve¡¡the¡¡remains¡¡before¡¡they¡¡had¡¡time¡¡to¡¡decay¡£¡¡On¡¡the¡¡other¡¡hand£»¡¡as¡¡long¡¡as¡¡the¡¡bed¡¡of¡¡the¡¡sea¡¡remained¡¡stationary£»¡¡thick¡¡deposits¡¡could¡¡not¡¡have¡¡been¡¡accumulated¡¡in¡¡the¡¡shallow¡¡parts£»¡¡which¡¡are¡¡the¡¡most¡¡favourable¡¡to¡¡life¡£¡¡Still¡¡less¡¡could¡¡this¡¡have¡¡happened¡¡during¡¡the¡¡alternate¡¡periods¡¡of¡¡elevation£»¡¡or£»¡¡to¡¡speak¡¡more¡¡accurately£»¡¡the¡¡beds¡¡which¡¡were¡¡then¡¡accumulated¡¡will¡¡have¡¡been¡¡destroyed¡¡by¡¡being¡¡upraised¡¡and¡¡brought¡¡within¡¡the¡¡limits¡¡of¡¡the¡¡coast¡­action¡£¡¡
Thus¡¡the¡¡geological¡¡record¡¡will¡¡almost¡¡necessarily¡¡be¡¡rendered¡¡intermittent¡£¡¡I¡¡feel¡¡much¡¡confidence¡¡in¡¡the¡¡truth¡¡of¡¡these¡¡views£»¡¡for¡¡they¡¡are¡¡in¡¡strict¡¡accordance¡¡with¡¡the¡¡general¡¡principles¡¡inculcated¡¡by¡¡Sir¡¡C¡£¡¡Lyell£»¡¡and¡¡E¡£¡¡Forbes¡¡independently¡¡arrived¡¡at¡¡a¡¡similar¡¡conclusion¡£¡¡
One¡¡remark¡¡is¡¡here¡¡worth¡¡a¡¡passing¡¡notice¡£¡¡During¡¡periods¡¡of¡¡elevation¡¡the¡¡area¡¡of¡¡the¡¡land¡¡and¡¡of¡¡the¡¡adjoining¡¡shoal¡¡parts¡¡of¡¡the¡¡sea¡¡will¡¡be¡¡increased£»¡¡and¡¡new¡¡stations¡¡will¡¡often¡¡be¡¡formed£»¡¡all¡¡circumstances¡¡most¡¡favourable£»¡¡as¡¡previously¡¡explained£»¡¡for¡¡the¡¡formation¡¡of¡¡new¡¡varieties¡¡and¡¡species£»¡¡but¡¡during¡¡such¡¡periods¡¡there¡¡will¡¡generally¡¡be¡¡a¡¡blank¡¡in¡¡the¡¡geological¡¡record¡£¡¡On¡¡the¡¡other¡¡hand£»¡¡during¡¡subsidence£»¡¡the¡¡inhabited¡¡area¡¡and¡¡number¡¡of¡¡inhabitants¡¡will¡¡decrease¡¡£¨excepting¡¡the¡¡productions¡¡on¡¡the¡¡shores¡¡of¡¡a¡¡continent¡¡when¡¡first¡¡broken¡¡up¡¡into¡¡an¡¡archipelago£©£»¡¡and¡¡consequently¡¡during¡¡subsidence£»¡¡though¡¡there¡¡will¡¡be¡¡much¡¡extinction£»¡¡fewer¡¡new¡¡varieties¡¡or¡¡species¡¡will¡¡be¡¡formed£»¡¡and¡¡it¡¡is¡¡during¡¡these¡¡very¡¡periods¡¡of¡¡subsidence£»¡¡that¡¡our¡¡great¡¡deposits¡¡rich¡¡in¡¡fossils¡¡have¡¡been¡¡accumulated¡£¡¡Nature¡¡may¡¡almost¡¡be¡¡said¡¡to¡¡have¡¡guarded¡¡against¡¡the¡¡frequent¡¡discovery¡¡of¡¡her¡¡transitional¡¡or¡¡linking¡¡forms¡£¡¡
From¡¡the¡¡foregoing¡¡considerations¡¡it¡¡cannot¡¡be¡¡doubted¡¡that¡¡the¡¡geological¡¡record£»¡¡viewed¡¡as¡¡a¡¡whole£»¡¡is¡¡extremely¡¡imperfect£»¡¡but¡¡if¡¡we¡¡confine¡¡our¡¡attention¡¡to¡¡any¡¡one¡¡formation£»¡¡it¡¡becomes¡¡more¡¡difficult¡¡to¡¡understand£»¡¡why¡¡we¡¡do¡¡not¡¡therein¡¡find¡¡closely¡¡graduated¡¡varieties¡¡between¡¡the¡¡allied¡¡species¡¡which¡¡lived¡¡at¡¡its¡¡commencement¡¡and¡¡at¡¡its¡¡close¡£¡¡Some¡¡cases¡¡are¡¡on¡¡record¡¡of¡¡the¡¡same¡¡species¡¡presenting¡¡distinct¡¡varieties¡¡in¡¡the¡¡upper¡¡and¡¡lower¡¡parts¡¡of¡¡the¡¡same¡¡formation£»¡¡but£»¡¡as¡¡they¡¡are¡¡rare£»¡¡they¡¡may¡¡be¡¡here¡¡passed¡¡over¡£¡¡Although¡¡each¡¡formation¡¡has¡¡indisputably¡¡required¡¡a¡¡vast¡¡number¡¡of¡¡years¡¡for¡¡its¡¡deposition£»¡¡I¡¡can¡¡see¡¡several¡¡reasons¡¡why¡¡each¡¡should¡¡not¡¡include¡¡a¡¡graduated¡¡series¡¡of¡¡links¡¡between¡¡the¡¡species¡¡which¡¡then¡¡lived£»¡¡but¡¡I¡¡can¡¡by¡¡no¡¡means¡¡pretend¡¡to¡¡assign¡¡due¡¡proportional¡¡weight¡¡to¡¡the¡¡following¡¡considerations¡£¡¡
Although¡¡each¡¡formation¡¡may¡¡mark¡¡a¡¡very¡¡long¡¡lapse¡¡of¡¡years£»¡¡each¡¡perhaps¡¡is¡¡short¡¡compared¡¡with¡¡the¡¡period¡¡requisite¡¡to¡¡change¡¡one¡¡species¡¡into¡¡another¡£¡¡I¡¡am¡¡aware¡¡that¡¡two¡¡palaeontologists£»¡¡whose¡¡opinions¡¡are¡¡worthy¡¡of¡¡much¡¡deference£»¡¡namely¡¡Bronn¡¡and¡¡Woodward£»¡¡have¡¡concluded¡¡that¡¡the¡¡average¡¡duration¡¡of¡¡each¡¡formation¡¡is¡¡twice¡¡or¡¡thrice¡¡as¡¡long¡¡as¡¡the¡¡average¡¡duration¡¡of¡¡specific¡¡forms¡£¡¡But¡¡insuperable¡¡difficulties£»¡¡as¡¡it¡¡seems¡¡to¡¡me£»¡¡prevent¡¡us¡¡coming¡¡to¡¡any¡¡just¡¡conclusion¡¡on¡¡this¡¡head¡£¡¡When¡¡we¡¡see¡¡a¡¡species¡¡first¡¡appearing¡¡in¡¡the¡¡middle¡¡of¡¡any¡¡formation£»¡¡it¡¡would¡¡be¡¡rash¡¡in¡¡the¡¡extreme¡¡to¡¡infer¡¡that¡¡it¡¡had¡¡not¡¡elsewhere¡¡previously¡¡existed¡£¡¡So¡¡again¡¡when¡¡we¡¡find¡¡a¡¡species¡¡disappearing¡¡before¡¡the¡¡uppermost¡¡layers¡¡have¡¡been¡¡deposited£»¡¡it¡¡would¡¡be¡¡equally¡¡rash¡¡to¡¡suppose¡¡that¡¡it¡¡then¡¡became¡¡wholly¡¡extinct¡£¡¡We¡¡forget¡¡how¡¡small¡¡the¡¡area¡¡of¡¡Europe¡¡is¡¡compared¡¡with¡¡the¡¡rest¡¡of¡¡the¡¡world£»¡¡nor¡¡have¡¡the¡¡several¡¡stages¡¡of¡¡the¡¡same¡¡formation¡¡throughout¡¡Europe¡¡been¡¡correlated¡¡with¡¡perfect¡¡accuracy¡£¡¡
With¡¡marine¡¡animals¡¡of¡¡all¡¡kinds£»¡¡we¡¡may¡¡safely¡¡infer¡¡a¡¡large¡¡amount¡¡of¡¡migration¡¡during¡¡climatal¡¡and¡¡other¡¡changes£»¡¡and¡¡when¡¡we¡¡see¡¡a¡¡species¡¡first¡¡appearing¡¡in¡¡any¡¡formation£»¡¡the¡¡probability¡¡is¡¡that¡¡it¡¡only¡¡then¡¡first¡¡immigrated¡¡into¡¡that¡¡area¡£¡¡It¡¡is¡¡well¡¡known£»¡¡for¡¡instance£»¡¡that¡¡several¡¡species¡¡appeared¡¡somewhat¡¡earlier¡¡in¡¡the¡¡palaeozoic¡¡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£»¡¡it¡¡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¡¡marine¡¡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¡¡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¡¡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£»¡¡might¡¡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¡¡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£»¡¡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¡¡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¡¡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¡¡palaeontologist£»¡¡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¡¡different¡¡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¡¡any¡¡idea¡¡of¡¡the¡¡time¡¡which¡¡its¡¡deposition¡¡has¡¡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¡¡suspected¡¡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¡¡t

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