IN ANT SINQL E FORMA TION. 32 7 



the world, sedimentary deposits, inchidincj fossil rcmaini^y 

 have gone on accumulating within tlie same ?\rea during 

 the whole of this period. It is not, for instance, pr()])ablo 

 that sediment was deposited during tlie whole of tlie 

 glacial period near the mouth of the Mississippi, within 

 that limit of depth at which marine animals' can best 

 flourish: for we know that great geograpliical 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 diiferent levels, 

 owing to the migrations of species and to geographical 

 changes. And in the distant future, a geologist, examin- 

 ing these beds, would be tempted to conclude that tlio 

 average duration of life of the 3mbedded fossils had been 

 less than that of tlie 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 continuously accumulating 

 during a long period, sufficient for the slow process of 

 modification; hence, the deposit must be a very thick 

 one; and the species undergoing change must have lived 

 in the same district throughout the whole time But 

 we have seen that a thick formation, fossiliferous 

 throughout its entire thickness, can accumulate oidy 

 during a period of subsidence; and to keej) Hie deptli 

 approximately the same, which is necessary that tiic 

 same marine species may live on the same space, the sup- 

 ply of sediment must nearly counterbalance the amount 

 of subsidence. But this same movement of subsi- 

 dence will tend to submerge tlie area whence the sediment 

 is derived, and thus diminish the supply, while tlie down- 

 ward movement continues. In fact, this neai-ly exact bal- 

 ancing 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 tlu* 



