METHODS OF CORRELATION • 1129 



Laramie formation. The standards of the North American marine 

 Eocenic and Oligocenic are found in the Gulf States, while those 

 of the Miocenic and Pliocenic are found on the Atlantic coast of 

 Maryland and Virginia, the Carolinas and Florida. All these locali- 

 ties together, however, furnish only a partial standard of the 

 American marine Tertiary. 



A Double Standard. In some cases it has been found most 

 practicable to have a double standard of formations : one marine, 

 the other non-marine. This has been most fully worked out for the 

 American Tertiary, where the continental deposits of the Great 

 Plains region serve as a standard of comparison for similar de- 

 posits ot other American regions, while the Gulf and Atlantic 

 Coast deposits serve as our standard of marine formations. In 

 western Europe the Carbonic is represented by the non-marine 

 Westphalian and Stephanian, while the corresponding marine forma- 

 tions of eastern Europe are the Moscovian and Uralian. In 

 America the Pocono-Mauch Chunk formations are the non-marine 

 equivalents of the Mississippic, and the Pennsylvanic (Pottsville- 

 Monongahela) is the non-marine standard of the Carbonic, the 

 marine standard being still undetermined, though in part represented 

 in the Kansas section. The Laramie, Belly River, Bear River, and 

 Dakota formations form a nearly complete non-marine standard for 

 the American Cretacic, and the Kootanie for the Comanchic. 



Methods of Correlation. 



The means by which formations of different localities are cor- 

 related may be summarized as follows : 



1. Superposition. 



2. Stratigraphic continuity. 



3. Lithic characters. • 



4. Organic contents. 



5. Unconformities or disconformities. 



6. Regional metamorphism. 



7. Diastrophism. 



I. Superposition. 



The basis of all stratigraphic work is the ascertainment of the 

 order of superposition of the strata. No correlation of the strata 

 in any two localities is possible until the exact superposition in 



