DISCRIMINATION OF TIME-VALUES IN GEOLOGY 573 

 TIME VALUES OF FORMATIONS. 



The regular sequence of stratified sediments forms a natural 

 geological column, which, in any particular section of the earth's 

 crust, is so conspicuously subdivided by lithologic differences in 

 kind of sediments that the divisions form the most satisfactory 

 kind of natural time division for geological classification. 



These natural, lithologic divisions of the crust of the earth 

 are technically called formations in the nomenclature of the 

 United States Geological Survey. And in any standard section, 

 such as that of New York state, the order, composition and thick- 

 ness of the several formations is exactly known ; and for that 

 section, too, the fossils of each separate formation are known 

 accurately and in large numbers. Geologists have been accus- 

 tomed to use such a local column of known geological formations 

 as a standard time scale ; and as examination has extended to 

 sections of the crust in other parts of the continent, the classifi- 

 cation and correlation of the other columns have been made to 

 correspond, by correlation, with such a standard column of 

 formations. Two methods have been used in establishing the 

 correlation: (i) by tracing continuity in the lithologic forma- 

 tion ; and (2) by recognition of identity of the fossil species con- 

 tained in the formations. Both of these methods have rested 

 on an assumed interpretation of the facts, the correctness of 

 which may be questioned quite independently of the established 

 fact of continuity or of identity. 



The assumptions on which these interpretations rest are that 

 correlations of time relations can be established in the first case 

 by continuity of litJiologic formation, and in the second case by 

 identity of fossils. In regard to the first case, it would be incor- 

 rect to say that the assumption is entirely false, for in some 

 cases, and to a limited extent, lithologic continuity of a forma- 

 tion is undoubtedly synonymous with sameness of the period of 

 the sedimentation represented by the formation. But the facts 

 are abundant, and well known to all field geologists, to prove 

 that formational continuity is not co-ordinate with lithologic 

 uniformity; and since our standard definition of a formation 



