IMPERFECTION OF PALAEONTOLOGICAL RECORD. 7 1 



beds much older than themselves.. These local unconformities are 

 exceedingly common in regions which have undergone much dis- 

 turbance, and they merely indicate that the region has been sub- 

 jected to a local elevation, resulting in a temporary cessation of 

 sedimentation in that particular area. In such cases, an examina- 

 tion of neighbouring areas, which remained submerged, and in 

 which, therefore, sedimentation was uninterrupted, will show the 

 missing deposits which were laid down during the period represented 

 in the first area by a local unconformity. The instances above 

 alluded to, though really only differing from the local unconformi- 

 ties just alluded to in nothing more than in being the result of a very 

 widespread elevation, are distinguished by an important point. In 

 the case of a mere local unconformity, we know what formation is 

 wanting, and we can intercalate it from foreign areas, and can thus 

 complete the series. In the general unconformities, such as that 

 between the Palaeozoic and Mesozoic groups of sediments, we are 

 not at present acquainted with the deposits which were laid down 

 during the interval represented by the physical discordance of the 

 strata, and the series of rock-formations thus remains a broken 

 one. 



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

 the imperfection of the palaeontological record is to be found in the 

 vast spaces of time which separate most of the great " systems," 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 allowed of great zoological 

 changes in the interim, and to have thus caused irremediable blanks 

 in the palaeontological record. The work of rock-deposition is at 

 best an intermittent process ; the changes in a fauna, if slowly 

 effected, are continuous. Thus there are scores of instances in 

 which the fauna of a given bed, perhaps but a few inches in thick- 

 ness, 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 interrupted 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 unrepresented time throughout the whole stratified 

 series is convincing proof that the palaeontological record is, and 



