314 REPORT OF NATIONAL MUSEUM, 1892. 



VI.— CORRELATIVE GEOLOGY AM) ITS CRITERIA. 



The term correlative geology is not in common use but it is .adopted 

 as a present convenience in discussing the correlation of assemblages 

 of strata as divisions or subdivisions of the geological scale as it is 

 developed in separate regions, and the identification of formations 

 within one and the same district or region. As here used the term 

 correlation refers to geological systems or other comprehensive series 

 of stratified rocks which occur in different and more or less widely sep- 

 arated parts of the world between which parts there is no physical 

 continuity of strata, or none find it is possible to discover. Correla- 

 tion applies to general geology, identification to local or regional inves- 

 tigations. 



The latter may be discussed under two heads, direct and relative. 

 Direct identification applies to formations the characteristics of which 

 at one or more localities have been ascertained, and as these are 

 naturally of limited geographical extent* the application is similarly 

 restricted. 



If a formation were exposed at the surface throughout its whole 

 geographiral extent its identity at all points would be self-evident, but 

 all formations being more or less covered from view by one another or 

 by surface debris, they are usually accessible for study only where 

 they have been corraded by drainage streams or brought to the sur- 

 face by movements of the earth's crust where their exposure has 

 been effected by subaerial erosion and denudation. It is at such 

 localities only that they can be satisfactorily identified, but such iden- 

 tification implies the actual or original continuity of the formation 

 between all the localities at which the identification has been made. 



It is the identification of formations and not their characterization 

 which is here discussed. The latter, as indicated in Essay n, must be 

 determined by original studies at one or more localities from a variety 

 of data, chief among which are the biological, although the physical 

 are always indispensable, while identification refers to a recognition of 

 those characteristics elsewhere. 



Specific identity of fossils affords the surest test of the direct identity 

 of a formation at localities between which its physical continuity can 

 not be traced, but lithological similarity, general evidence of liomogeny 

 and other physical indications are often eflieient aids in such identifica- 

 tion, and in case of failure of biological evidence they are often in a 

 good degree satisfactory. 



By the term relative identification is meant the recognition of the 

 proper place of a given formation in a series the taxonomic order of 



* The term formation is herein used in accordance with the restricted definition 

 and the characterization which are given in Essay II. 



