414 STUART WELLER 



beds were laid down, the older basin extending much farther 

 north and reaching westward into northern Missouri and Iowa, 

 probably surrounding or submerging the Ozark land. The sea- 

 pattern, then, of Chester time was quite different from that of 

 the earlier Mississippian. The succession of sandstones and lime- 

 stone-shale formations laid down in this basin in Chester time 

 indicate a series of oscillations of the sea occupying the basin, 

 comparable in a way to the oscillations in the much larger basin of 

 Lower Mississippian time. 



Throughout the alternating succession of Chester formations, 

 the several units should be considered in pairs, each pair consisting 

 of a sandstone formation below, passing upward into a Hmestone- 

 shale formation. In a number of horizons in the series the sand- 

 stone formation clearly exhibits an unconformable contact with the 

 underlying limestone, but in no case is there any evidence of 

 unconformity between the sandstone and the overlying Hmestone- 

 shale unit. Each one of these pairs doubtless represents one 

 oscillatory advance and retreat of the waters of the basin, the 

 lower sandstone unit in the pair being a transgressing formation 

 associated with the advancing submergence, the limestone and 

 shale deposition lagging behind the sand accumulation at a greater 

 distance from the shore line. In so far as the sandstones He un- 

 conformably upon the underlying limestone the magnitude of the 

 oscillation has been sufficient to cause the waters of the- basin to 

 withdraw to a position south of the localities where observations 

 upon surface outcrops have been possible. If the horizons of such 

 unconformities could be traced southward toward the open sea of 

 the period, they would presumably pass into entirely uninterrupted 

 series of sediments, and if they could be followed still farther in 

 the same direction the sandstone members in the succession of 

 beds should disappear and a continuous limestone formation 

 should represent the Chester series. At those horizons where no 

 evidence of unconformity between the sandstone and the under- 

 lying limestone exists, the southernmost position of the retreating 

 shore line of the basin was presumably somewhat north of the 

 localities where the outcrops have been observed, and if the posi- 

 tion of the ancient shore line at a period of emergence crossed the 



