i893. NOTES AND COMMENTS. 407 



(5) The elaboration of the Mississippian series, or "sub- 

 Carboniferous " formations of the Mississippi River basin. 



(6) The Permian problems of Kansas and Nebraska. 



(7) The correlation problems involved in classifying (a) the 

 formations of the Acadian province, and (b) the formations of the 

 Rocky Mountains and Western Plateau provinces. 



In the discussion of these various problems, the definite stages in 

 the development of the principles of correlation have been recognised. 



At the opening of the century, the Wernerian system of classi- 

 fication was adopted. In this classification, the mineral character 

 of the formations was regarded as fundamental. 



The second stage took definite shape in the New York system, 

 and while a general " parallelism of strata " was believed in, " gaps " 

 and " intercalations " were assumed, to make the interpretations fit the 

 facts. Fossils played a secondary part, only being considered of value 

 when exact identity was recognised. This principle did not reach 

 satisfactory results, because stratigraphic order and stratification 

 itself offer no intrinsic evidence of the age of a formation, and 

 stratigraphic structure w r as found not to be uniformly persistent even 

 for a few miles' extent. 



William Smith, early in the century, advanced the idea that 

 strata could be identified by the fossils they contained, and we need 

 not trouble the reader with any proofs of the value of this. 



Hence in the third stage of the correlations methods, fossils 

 assumed the chief role, and the minute and exhaustive study of 

 organised beings in their stratigraphical and geological relations has 

 proved to be, not merely the best, but the only reliable guide to 

 correlation of geological formations. 



The conclusions reached from these historical studies confirm the 

 belief that the description and nomenclature of structural forma- 

 tions should be quite independent of their correlations, and that 

 precision in correlation must be based upon mature and exhaustive 

 palaeontological study, that the time-scale must be made independently 

 of the structure-scale, and that the time-scale of correlation is based 

 fundamentally upon biological data. 



The investigations also lead to the further conclusions that, as 

 nomenclature finds its basis in some intrinsic characters of the things 

 named, uniformity of nomenclature for formations is impracticable, since 

 the intrinsic characters of formations are local, and have nothing to do 

 with their geological position; and that uniformity of classification can 

 be looked for only through an exhaustive biological study of fossils, 

 and is inapplicable to geological structure, stratigraphy, or formation. 



When it is mentioned that the Archaean paper occupies 550 

 closely printed octavo pages, it will be seen that the amount of labour 

 expended on these Reports is prodigious. They are most valuable to 

 the geological student on both sides of the Atlantic, and we regret that 

 space does not allow us to give a detailed account of the several 

 monographs. 



