Chap. IX. GEOLOGICAL RECORD. 295 



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 accumulating for a very long 

 period, in order to have given sufficient time for the 

 slow process of variation ; hence the deposit will gene- 

 rally have to be a very thick one ; and the species un- 

 dergoing modification will have had to live on the same 

 area throughout this whole time. But we have seen 

 that a thick fossiliferous formation can only be accumu- 

 lated 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. When 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 



