CORRELATION 579 



Marine deposits, as is well known, are accumulated under much more 

 uniform conditions than continental deposits, and hence are much more 

 constant in composition and thickness than the latter, which, in com- 

 parison to them, possess a high degree of variability. As shown by 

 many cases, the assumption of constancy of intervals between marine 

 sediments would be untrustworthy in regions as remote as western Penn- 

 sylvania and Maryland without independent evidence. The assumption 

 of constancy of interval between land sediments manifestly needs still 

 more evidence. 



The assumption of constancy of structure of coal seams is practically 

 the affirmation of the constancy of physical conditions over large arctis 

 and is open to still greater objections. The authors would, indee<], 

 question the continuity of many coal seams over such areas. Other 

 types of sediments are even more variable than coals. 



However valuable these criteria are within short distances, they are 

 evidently insufficient when employed over great distances unless fortitied 

 by more cogent considerations. The authors have, therefore, sought to 

 correlate the strata by means of their contained organisms, aided by the 

 consideration of lithology, coal structures, systematic variations of inter- 

 vals, sequence of deposits, and physical conditions of sedimentation. 



This study has led to t"he recognition of a critical series of strata 

 which can be distinguished by these criteria, and hence can be traced 

 throughout large areas in the northern Appalachian coal fields. They 

 furnish the key to the correlation of the middle Coal Measures in the 

 area under consideration. 



Characte7' of the critical series of tlie Lower Conemaugli. — The critical 

 series of beds is found in the lower part of the Conemaugh formation and 

 is characterized as follows : 



Faunas. — It embraces four horizons bearing marine faunas, named in 

 descending order : 



To]>. 



Ames limestone and fauna. 



Portersville limestone and fauna. 



Cambridge limestone and fauna. 



Brush Creek limestone and fauna. 

 Bottom. 



The Ames limestone, the upper member of the series, forms lenticular 

 beds of limestone or occurs as discontinuous limestone nodules in a dark, 

 fossiliferons shale. It bears a profuse fauna which is characterized by 

 containing great numbers of Amhocoelia planoconvexa, Clionetes granu- 



