IMPERFECTION OF GEOLOGICAL RECORD 75 



must nearly counterbalance the amount of subsidence. 

 But this same movement of subsidence will tend to sub- 

 merge the area whence the sediment is derived, and thus 

 diminish the supply, while the downward movement con- 

 tinues. 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 paleontologist 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 in- 

 spection of a formation 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 elsewhere 

 thousands of feet in thickness, and which must have re- 

 quired an enormous period for their accumulation; yet 

 no one ignorant of this fact would have even suspected 

 the vast lapse of time represented by the thinner forma- 

 tion. Many cases could be given of the lower beds of a 

 formation having been upraised, denuded, submerged, and 

 then recovered by the upper beds of the same formation 

 — facts, showing what wide, yet easily overlooked, inter- 

 vals have occurred in its accumulation. In other cases 

 we have the plainest evidence in great fossilized trees, 

 still standing upright as they grew, of many long inter- 





