CONTEMPORANEITY OF STRATA. 49 



troversy whether they should be regarded as the summit of 

 the older or the base of the newer series of sediments. 



We may pause here to consider how it is that we may 

 never hope to find a complete series of deposits linking on 

 one great formation to another, as, for example, the Chalk to 

 the Eocene rocks. In the first place, only a limited portion 

 of the earth has as yet been properly examined, and we have 

 therefore no right to expect that we have as yet hit upon 

 the area, or areas, to which the process of rock-forming was 

 transferred at the close of the Cretaceous period proper in 

 Europe. We have, however, the full right to expect that 

 we shall ultimately find formations which will have to be 

 intercalated in point of time between the White Chalk and 

 the Eocene ; and, as before said, traces of such are already 

 known to us. In the second place, we have every reason to 

 suppose that many of these intermediate deposits have been 

 destroyed at some period subsequent to their formation by 

 what is technically called " denudation," or, in other words, 

 by the action of rain, rivers, ice, and the sea. In the third 

 place, many of the missing deposits may have been concealed 

 since their formation by the deposition upon them of other 

 newer rocks ; or they may be situated in areas which are at 

 present covered by the ocean. Lastly, we must not forget 

 that there may have been times in which great changes in 

 life were actively progressing in areas in which there might 

 be little or no contemporaneous deposition of rock, so that 

 the extreme terms of a series might be preserved to us whilst 

 all the intermediate links might have escaped record. 



From these and similar causes, it is almost certain that we 

 shall never be able to point to a complete series of deposits 

 linking one great geological period, such as the Cretaceous, to 

 another, such as the Eocene. Still, we may well have a 

 strong conviction that such deposits must exist, or must have 

 existed, as memorials of, at any rate, part of the time which 

 elapsed between the close of the one formation and the com- 

 mencement of the next. Upon any theory of " evolution," 

 at any rate, it is certain that there can be no total break in 

 the great series of the stratified deposits, but that there must 

 have been a complete continuity of life, and a more or less 



VOL. I. D 



