Bulletin 28 158 



use in pioneer work, but should be replaced by the more accu- 

 rate evidence furnished by the identity or similarity of 

 faunas. In 1912, Clark and Miller* referred the beds at Peters- 

 burg and those mentioned above on the Pamunkey river to the 

 Calvert stage, and the remainder of the Murfreesboro stage as 

 here defined to the St. Mary's. It should not be understood 

 that no Calvert or St. Mary's is known in Virginia, as both are 

 well represented in the northern or northeastern part of the 

 state. In North Carolina, the stage includes all of the so-called 

 St. Mary's of the same workers. f 



Age Determination and Paleontology 



Stratigraphically, the stage occupies a position immediately 

 below the Yorktown, apparently conformable with it and thus in 

 a manner its upper age limit is not sharply defined. For more 

 precise age determination, the fossil evidence must be considered, 

 and in itself is conclusive. An analysis of the Murfreesboro 

 fauna corroborates the stratigraphic evidence and indicates a 

 much closer relationship with the Yorktown than with the St. 

 Mary's, or older beds. A considerably larger percentage of spe- 

 cies are common to the Yorktown than to the Marylandian Mio- 

 cene and the whole aspect of the fauna is of a more recent and 

 advanced type. Several of the species of mollusks which have 

 continued up from the lower and Middle Miocene show notice- 

 able evolutionary changes, correlative with time, being either 

 more, perfect or having assumed senile characteristics. An ex- 

 ample which might be cited is Glycymeris subovatus, a species 

 commencing first in the upper Oligocene, where it is small, con- 

 vex and rounded in outline. It is similar but rare in the lower 

 and Maryland Miocene, becomes large and very abundant in the 

 Murfreesboro, generally losing its convex form and becoming de- 

 pressed with senility, indicated by an increase in thickness of the 



*Virginia Geological Survey, Bull. 4. 



jNorth Carolina Geological and Economic Survey, vol. 3. 



