IMPERFECTION OF GEOLOGICAL RECORD 265 



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 counter- 

 balanced 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. ^VTien we see, 

 as is so often the case, a formation composed of beds 

 of different mineralogical composition, we may reason- 

 ably 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 deposi- 

 tion has consumed. Many instances could be given of 

 beds only a few feet in thickness, representing forma- 

 tions, 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 trees, still standing upright 



