PAllEONTOLOGICAL KECORD. 387 



■wMcli are not thoroughly established, are those between the Lower 

 and Upper Laurentian, the Upper Laurentian and Huronian, and 

 the Upper Cambrian and Lower Silurian. 



It may be well to point out that the unconformabilities here indi- 

 cated must in no way be confounded with the common cases in which 

 beds of one age rest unconformably upon beds far older than them- 

 selves. "When, for example, we find beds of Carboniferous age rest- 

 ing unconformably upon Silurian Rocks, this merely indicates that 

 in the particular locality under examination the Devonian or Old 

 Hed Sandstone is missing. This absence of a whole formation in 

 any given region merely shows that the afea was dry land during the 

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

 deposited in this locality, they were removed by subsequent denuda- 

 tion. Here, however, we know what formation is wanting, and we 

 can intercalate it from areas elsewhere, and thus complete the series. 

 The case is very difierent in the instances above spoken of, as where 

 the Permian Rocks rest unconformably upon the Carboniferous. 

 Here, 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 palseontological record is to be found in the 

 vast spaces of time intervening between most of the great formations, 

 not represented, so far as we yet know, by any formation of rock. 

 In process of time we shall doubtless succeed in finding deposits for 

 some of this uru'epresented 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 pavises 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 palseontological record. Thus, 

 there are hundreds of instances in which the fauna of a given bed, 

 perhaps but a few inches thick, difiers altogether from that of the 

 beds immediately above or below, and is characterized 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 sedi- 

 ment was laid down, whilst time was allowed for the dying out of 



