NEW YORK ACADEMY OF SCIENCES 59 1 



passed in review, chiefly along the work of I. H. Ogilvde. With 

 a brief statement of the postglacial lake- fillings, etc., which 

 have been especially set forth by C. H. Smyth, Jr., the paper 

 closed. 



A brief discussion followed. 



Dr. Berkey said that both Cambric and Ordovicic strata 

 contain prominent sandstone formations alternating with 

 dolomites wherever exposed in Michigan, Minnesota, Wis- 

 consin, Iowa, Illinois, Missouri, Arkansas, and Indian Territory. 

 The northern margin, however, is prevailingly more arenaceous 

 than the southern, where shales replace many sand beds. At 

 still greater distance, in Ohio, Kentucky, and Tennessee, these 

 are in turn represented by limestones largely. 



The uppermost one of the series is the St. Peter. This sand- 

 stone, as well as each of the more important ones below, is 

 believed to represent an extensive retreat and readvance of 

 the sea. Few marks of the erosion interval are preserved. 

 Only here and there has the mantle of sand permitted much 

 attack upon the underlying dolomite, and the reworking of 

 the sands themselves has obliterated most internal evidence 

 of such history. 



Much of the sand, furthermore, is wind-blown. The rework- 

 ing by the sea and the wind is believed to be the chief cause 

 of the extreme purity of the St. Peter. 



The St. Peter stage of the Ordovicic, therefore, represents a 

 retreat of the Mississippian Sea from the vicinity of Lake Supe- 

 rior to probably as far as Ohio, southern Illinois, and Arkansas, 

 followed by a readvance to nearly its original position. The 

 northern part of the St. Peter contains within itself therefore 

 a sedimentary break. In part it is both older and younger 

 than the same formation in its southern extension, while, on 

 account of the reworking accompanying the sea advance, 

 there is greater conformity with overlying than with underlying 

 beds. 



Dr. Berkey's paper was followed by a brief discussion, after 

 which the Section adjourned. 



A. W. Grabau, 



Secretary. 



