66 INTRODUCTION. 



i 



foreign areas, and thus complete the series. In the latter 

 case we have two successive formations in unconformable 

 junction, and we are not acquainted with any intermediate 

 group of strata which could be intercalated from any other 

 locality. 



From the above facts, then, we learn that one of the chief 

 causes of the imperfection of the palasontological record is 

 to be found in the vast spaces of time which separate most 

 of the great " formations," and which, so far as we yet know, 

 are not represented by any formation of rock. In process 

 of time we shall doubtless succeed in finding deposits to 

 account for more or less of this " unrepresented time/' but 

 much will ever remain for which we cannot hope to find 

 the representative sediments. It only remains to add that 

 we have ample evidence within the limits of each formation, 

 and wholly irrespective of any want of conformity, of such 

 lengthened pauses in the work of deposition as to have al- 

 lowed of great zoological changes in the interim, and to have 

 thus caused irremediable blanks in the palseontological record. 

 The work of rock-deposition is at best an intermittent pro- 

 cess ; the changes in a fauna, if slowly effected, are continu- 

 ous. Thus there are scores of instances in which the fauna 

 of a given bed, perhaps but a few inches in thickness, differs 

 altogether from that of the beds immediately above and 

 below, and is characterised by species peculiar to itself. In 

 such cases we can only suppose, that though no physical 

 break can be detected, the deposition of sediment was inter- 

 rupted by pauses of incalculable length, during which no 

 additional material was added to the sea-bottom, whilst time 

 was allowed for the dying out of old species and the coming 

 in of new. The incessant repetition of such intervals of un- 

 represented time throughout the whole stratified series is 

 convincing proof that the palaeontological record is, and ever 

 must be, a mere excerpt from the biological annals of the 

 globe. 



III. THINNING OUT OF BEDS. Another cause by which 

 the continuity of the palseontological record is affected is 

 what is technically called the " thinning out " of beds. 

 Owing to the mode in which sedimentary rocks are produced, 



