IMPERFECTION OF PAL^ONTOLOGICAL RECORD. 35 



Rocks. All these physical breaks are accompanied by more 

 or less extensive palaeontological breaks as well. Other breaks 

 which are rendered less important by the absence or scarcity 

 of fossils, or which are as yet not thoroughly established, are 

 those between the Lower and Upper Laurentian Rocks, the 

 Upper Laurentian and Huronian, and the Upper Cambrian 

 and Lower Silurian. 



It may not be out of place to point out that the unconforma- 

 bilities here indicated must in no way be confounded with the 

 common cases in which beds of one age rest unconformably 

 upon beds far older than themselves. When, for example, we 

 find beds of Carboniferous age reposing unconformably upon 

 Silurian strata, this merely indicates that, in the particular lo- 

 cality under examination, the Devonian or Old Red Sandstone 

 is amissing. This absence of a whole formation in any given 

 region merely indicates that the area was dry land during the 

 period of that formation, or that if any rocks of this age were 

 deposited in this locality, they were removed by denudation 

 before the higher group was laid down. The instances above 

 spoken of, as where the Carboniferous Rocks are succeeded 

 unconformably by the Permian, though essentially of the same 

 nature, are distinguished by an important point. In the former 

 case we know what formation is wanting, and we can intercal- 

 ate it from 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 palaeontological 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 deposi- 

 tion 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 I 



