PHYSICAL AND FAUNAE EVOLUTION 85 



in central Ohio; while the highest beds rest on Monroan or even 

 Niagaran, in southern Ohio. Continuing southeastward, the black 

 shale rises in the series, until in eastern Tennessee it is of Lower 

 Mississippic age, and rests on Lower Siluric or on Ordovicic strata.^ 

 (Fig. 11). 



DISCUSSION 



PROFESSOR CALVIN 



I have studied the St. Peter sandstone in Iowa, Wisconsin, Minnesota, and 

 IlHnois, and nowhere have I seen any marl^ed indications of cross-bedding such 

 as would be consistent with an aeoHan origin of the formation. In Iowa and 

 Minnesota there are few structural bedding planes seen in fresh sections, but those 

 that do exist are always horizontal and parallel. Bedding planes are more numer- 

 ous in this sandstone west of Ottawa, 111., but they are all precisely of the character 

 one sees everywhere in aqueous sediments. When the St. Peter is exposed on 

 sloping hillsides, by a process akin to exfoliation, it breaks off in thick flakes 

 parallel to the exposed surface and so often presents a false appearance of cross- 

 bedding; but this feature has no relation to the original structure. One hardly 

 needs to go to the Libyan desert to ascertain the characteristics of aeolian sands. 

 The region around the south end of Lake Michigan affords ample opportunity, 

 nearer home, to study the structural features and topographic forms of wind- 

 blown deposits. I have seen nothing in the St. Peter suggesting similar origin. 

 Furthermore, the St. Peter occasionally contains marine fossils, as shown by 

 Winchell and Sardeson. 



REJOINDER 



PROFESSOR GRABAU 



Most writers on the St. Peter have described it as showing marked cross- 

 bedding. It is not always readily recognized and is often overlooked in sand- 

 stones of this type. But even if there were no cross-bedding in the known 

 exposures, this would be no argument against the eolian origin of the formation, 

 since modem eolian deposits often lack such a structure — and, indeed, sometimes 

 lack all bedding. The comparison with the sands of the Libyan desert is made 

 because they represent precisely what is believed to be the origin of the St. Peter 

 and Sylvania. These sands are derived from an older sandstone — not from 

 glacial sands as are those of Lake Michigan. They show a similar purity, 

 rounding, and uniform size as found in the St. Peter and Sylvania. Finally 

 they have been transported a great distance and rest with a sharp contact on 

 eroded limestones, often including the weathered-out fossils of this limestone in 

 their base. The fossils obtained from the St. Peter were, as noted in the te.xt, 

 obtained from the lower and upper parts, the latter of the Stones River type, 

 included in the rewashed upper sands on the advance of the Stones River sea 

 over the old dune area. The fossils included in the basal part are either 

 weathered-out Beekmantown or included in the sands during the retreatal phase. 

 Such inclusions are to be expected in deposits of this type. 



I See Grabau, "Types of Sedimentary Overlap," Bull. Geol. Soc. Amer., Vol. 

 XVII, pp. 593-613. 



