CuAP. X.] IN ANY SINGLE FORMATION. 69 



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 paleeontologist, 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 formation give us any idea of the length 

 of time wliich 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 wliich must 

 have required an enormous period for their accumu- 

 lation ; yet no one ignorant of this fact would have 

 even 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 trees, still standing upright 

 as they grew, of many long intervals of time and 

 changes of level during the process of deposition, which 



