mo PRINCIPLES OF STRATIGRAPHY 



locality, both should be called by the same name. Such exact 

 equivalency, however, seldom obtains. The following figures 

 copied from Willis' paper (20) show the case mentioned and 

 the far more common cases in which such depositional equiva- 

 lency is not complete. In Diagram II the m (shale) formation 

 grades into the n (limestone) formation, but with a prolonged over- 

 lap. In this case neither formation is the exact equivalent of the 

 other, and both may occur together. Hence, each should receive 

 a dififerent name. An example of this kind is furnished by the 



Fig. 262. Diagrams showing horizontal variation in sediments. (After 

 Willis.) 



Catskill and Chemung formations, which grade into each other by 

 overlap, the Catskill alone being present in eastern New York and 

 the Chemung alone in western New York, while between these 

 points parts of both are present. In the diagram cited, the near 

 shore overlapping the ofifshore deposits, the overlap is regressional 

 and a replacing one and due to shoaling of the water. In the Cat- 

 skill-Chemung case, a continental formation overlaps a marine one. 

 Diagram III represents three formations on the right equivalent 

 to the shale formation (m) on the left. This shale formation (;;;) 

 is represented on the right by its middle portion, while the lower is 

 replaced by a sandstone formation and the upper by a limestone 

 formation. Each of the two new formations receives a distinct 

 name, as p sandstone formation and s limestone formation. If the 

 name "w shale" is retained for the middle member, a new name 

 (x) for the entire group p m s must be given, the x group being 

 then equivalent to the m shale of the left hand locality, but in- 

 cluding the ;;; shale at the right hand locality. A better method, 

 however, is to give the shale on the right hand a new name (k) 

 and call the group p k s the m group, this being equivalent to the 



