CHAPTER XXXII. 



CORRELATION: ITS CRITERIA AND PRINCIPLES— 

 PAL^OGEOGRAPHY. 



Correlation. 



"The fundamental data of geologic history are: (i) local se- 

 quences of formations, and (2) the chronologic equivalences of 

 formations in different provinces. Through correlation all forma- 

 tions are referred to a general time scale, of which the units are 

 periods. The formations made during a period are collectively des- 

 ignated a system." (Rule 14, Nomenclature and Classification for 

 the Geologic Atlas of the United States.) 



History of Development of Methods of Correlation. 



Correlation of strata, or the establishment of an orderly relation- 

 ship between the formations of separate regions, has been one of 

 the chief aims of stratigraphers ever since the days of Werner and 

 William Smith. Werner's correlations were based on the lithic 

 character of the strata, but William Smith in England and Cuvier 

 and Brogniart in France made their identifications of strata by 

 means of the organic remains included in them. Each of these 

 workers based his investigation upon the ascertained succession 

 of strata in the region selected by him as typical, and thus the 

 three fundamental criteria of correlative geology : lithic similarity, 

 likeness of fossil content and superposition of strata, were made 

 use of by the pioneers in stratigraphy. 



The efforts of these founders of stratigraphy were directed 

 chiefly toward establishing the identity or correspondence of strata 

 between different localities ; and, when it was recognized that strata 

 were formed at different periods in the earth's history, the effort 

 was further directed toward establishing the time-equivalency or 

 synchroneity of strata. Before fossils were extensively studied, 



