Chap. X.] IN ANY SINGLE FORMATION. 277 



these beds, would 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 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 only during a period of subsidence ; and to keep the 

 depth approximately the same, which is necessary that the same 

 marine species may live on the same space, the supply of sediment 

 must nearly counterbalance the amount of subsidence. But this 

 same movement of subsidence will tend to submerge 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 widely different mineralogical composition, we 

 may reasonably suspect that the process of deposition has been 

 more or less interrupted. Nor will the closest inspection of a for- 

 mation give us any idea of the length of time which its deposition 

 may have consumed. Many instances could be given of beds only 

 a few feet in thickness, representing formations, which are else- 

 where thousands of feet in thickness, and which must have required 

 an enonnous period for their accumulation ; yet no one ignorant of 

 this fact would have even suspected the vast lapse of time repre- 

 sented by the thinner formation. Many cases could be given of 

 the lower beds of a formation having been upraised, denuded, sub- 

 merged, and then re-covered by the upper beds of the same forma- 

 tion, — 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 not have been sue>- 



