298 T. O. MABRY 



with several feet of loam, which becomes thicker down the hill- 

 side and in the adjacent valley ; near the hilltop the underlying 

 Lafayette sands contain scattered quartz pebbles, while further 

 down the hill, at a considerably lower level there is a well- 

 defined pebble stratum at the base of the loam. Toward the 

 hilltop the pebbles are evidently well within the Lafayette, 

 while toward the bottom of the hill the pebble stratum seems to 

 form the basal member of the Loam, though it is undoubtedly 

 derived from the higher level gravel of the Lafayette (compare 

 the relations of the Loess to certain gravels in southern Illinois 

 Am. Jour. Set., Vol. XLI, p. 366 et seq.). 



Similar gravels have been described by Professor Salisbury 

 (Jour. Geol., Vol. Ill, pp. 655-667), from Devil's Lake, Wis- 

 consin, where they underlie the Drift. They are therefore of Pre- 

 Glacial age. Direct correlation of this deposit with the southern 

 gravels is at present impossible, but it seems probable that both 

 were laid down by the same "definitely limited set of agencies" 

 acting within "a definitely limited period of time" — a period 

 closed by the inauguration of the Glacial period in the North. 



As to the conditions under which the Lafayette was depos- 

 ited, I do not feel prepared to speak. However, it seems to me 

 that Hilgard's view as stated in "The Age and Origin of the 

 Lafayette Formation" (Am. Jour. Set., No. 257, Vol. XLIII), 

 on the whole, is to be preferred ; only we must look to another 

 source than melting continental glaciers, for the floods which 

 brought down and deposited the materials of the Lafayette. 



B. Upper limit oj the Loess-Ltam.— This formation covers by 

 far the greater part of the surface in this region, and it is only 

 in the "second bottoms" and in the bottoms proper that we find 

 materials of a possibly later age. Many of these " second 

 bottoms" are simply low, broad terraces of degradation carved 

 out of the Yellow Loam, as already noticed. Others are proba- 

 bly stream terraces of constructive origin. But as such deposits 

 are confined to the vicinity of streams, deposition along 

 streams proceeding pari passu with erosion of the general sur- 

 face of the country ; and because the materials of such terraces 



