1 132 PRINCIPLES OF STRATIGRAPHY 



This limestone, intercalated between black shales, indicates a pe- 

 riod of widespread uniform conditions, followed by a resumption 

 of the Black Mud sedimentation. It therefore serves as an excel- 

 lent horizon marker, by which the rocks above and below can be 

 correlated. Still another example is the Cobleskill of New York, 

 which has been traced across the State, chiefly by its fauna, and 

 serves as a datum-plane for the strata above and below it. 



3. LitJiic characters. 



Correlation by lithic characters is possible only in very limited 

 areas, and where it can be used in connection with stratigraphic 

 continuity and order of superposition. Under certain conditions, 

 however, the lithic character becomes an important guide in cor- 

 relation. An example is the St. Peter sandstone, a pure silicarenyte, 

 which has been widely recognized by its lithic character, and its 

 enclosure within pure limestone or dolomytes. The uniformity of 

 grain and composition over thousands of square miles of area 

 is its most remarkable feature. As already outlined, however 

 (Chapter X\TII), the St. Peter, though occupying a definite posi- 

 tion in the scale, encloses within itself a hiatus, which constantly 

 widens northward, so that the top of the sandstone is higher in 

 the scale and the bottom is lower in the northern region, as com- 

 pared with its more southern occurrence. Aloreover, there are 

 formations, such as the Sylvania sandstone of Ohio (Upper Sil- 

 uric), which are lithically identical with the St. Peter, and might 

 be mistaken for it, if lithic character alone were considered. 



An intercalated shore-derived formation between offshore 

 formations can generally be recognized in its various outcrops by 

 its lithic character. Such a formation represents an oscillation of 

 the land during sedimentation, either a shoaling or a total retreat 

 of the sea, followed by a re-advance or a deepening. Lithic charac- 

 ter then, when taken in conjunction with superposition, may be 

 a valuable guide in correlation. Intercalated off-shore beds among 

 terrigenous formations may likewise serve a good purpose in cor- 

 relation. Thus the Ames or Crinoidal limestone, a marine bed, has 

 been widely recognized as an intercalated bed in the non-marine 

 Conemaugh formation of the bituminous coal field. In this case 

 the correlation is confirmed by the contained fauna. 



Another way in which lithic character serves a useful purpose 

 in correlation is by the occurrence of what may be called sympa- 

 thetic changes in sedimentation. Thus two regions, one more dis- 



