1,1 T. C. CHAMBERLLN — THE [NTERGLACIAL INTERVAL. 



Conclusion. — It would appear, therefore, that while there are local varia- 

 tions there is a general correspondence between the amount of erosive work 

 done by the lower Mississippi, by the upper Ohio and Allegheny, by the 

 Susquehanna, and by the Delaware rivers respectively. The facts indicate 

 that the altitude of the continent was low in the closing stages of the earlier 

 glacial epoch; that it became higher in the'interglacial interval; and that 

 after sufficient time elapsed for these great erosious to take place, the glacial 

 water.- of the later epoch poured their valley deposits down the trenches formed 

 in the interval. The cutting of these trenches rudely measures the length of 

 this interval, or at least the length of the actively erosive part of it. 



DISCUSSION. 



.Mr. W .1 McGee: President Ghamberlin remarks that the orange sands 

 of the south are largely preglacial or Tertiary. Now " Orange Sand " is 

 the name of a series of dep »sits grouped and so designated many years ago 

 by Professor E. W. Hilgard. That series really includes deposits of widely 

 diverse ages: Beginning with the newest, it includes certain Pleistocene 

 or glacial gravels forming the basal member of the loess; it includes also 

 the gravels of a wide-spread deposit elsewhere termed the Appomattox forma- 

 tion; it includes, too, certain gravels and loams which are early Cretaceous, 

 or possibly Jurassic — the Potomac formation, or the Tuscaloosa of Smith 

 and Johnson. In addition to these deposits of definitely determined ag 

 it includes a variety of residuary gravels and Loams which extend from the 

 present hack to the close of the Jurassic. By far the greater part of the 

 "Orange Sand" consists of materials properly included in the Appomattox 

 formation, and the greater pari of the remainder consists of materials 

 which are earlier than Pleistocene. But I desire to call special attention to 

 certain Pleistocene gravels, heretofore classed with the "Orange Sand," 

 which it seems to me that President Chamberlin has overlooked. They 

 occur in part- of the lower Mississippi region, uotably in the neighborhood 

 of Vicksburg and Grand Gulf, Mississippi. There may he found a magnifi- 

 cent developmenl ofloess, which is charged with fossils and is in all respects 

 so characteristic that these localities may he regarded a- typical for the 

 loess of tie- North American continent. This loess rests ou the gravel in 



qU68tion. Now careful examination -how- that the loess and gravel are not 



unconformable, a- hitherto supposed, hut that the one graduates into the 

 other. This in tergradation takes place by interstratification ; the loess firel 

 becomes sandy al the base, and then becomes interstratified with silts; and 

 -till lower the loess appears only in thin layers interbedded with silts, loams, 



-and-, ami finally gravels. Thi- stratum of tran-iti nay he 10 or 15 feet 



in thickness; hut there is absolutely imperceptible transition by interstrati- 



