IMPERFECTION OF PAL^ONTOLOGICAL RECORD. 65 



characterised by a transitional series of fossils, lying between 

 the highest undoubted Cretaceous and the lowest unques- 

 tioned Tertiary strata. The deposits in question the so- 

 called " Lignitic Series " are very thick, and the inter- 

 mixture of Secondary and Tertiary types of life which they 

 exhibit is so complete, that it has been found a matter of 

 great difficulty to assign them definitely either to the 

 Cretaceous or to the Eocene. 



Amongst other well-known instances of more or less 

 general unconformity in the stratified series, may be men- 

 tioned that between the Lower and Upper Silurian (not al- 

 ways present), that between the Lower and Upper Old Eed 

 Sandstone (also not universal), that between the Carbonif- 

 erous and Permian rocks, that between the Permian and 

 Triassic rocks (not universal), and that between the Lower 

 and Upper Cretaceous 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 Huro- 

 nian, and the Upper Cambrian and Lower Silurian. 



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

 formabilities here indicated must in no way be confounded 

 with the common cases in which beds of one age rest uncon- 

 formably upon beds far older than themselves. When, for 

 example, we find beds of Carboniferous age reposing uncon- 

 formably upon Silurian strata, this merely indicates that, in 

 the particular locality under examination, the Devonian or 

 Old Ked 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 distin- 

 guished by an important point. In the former case we know 

 what formation is wanting, and we can intercalate it from 



VOL. I. E 



