Pleistocene to P re-pleistocene of Mississippi Basin. 361 



The shells, the concretions (loess-Jcindchen), and the calcareous 

 nature of the loess, are features not less characteristic, though 

 less universal, than the texture itself. As the texture of the 

 loess becomes closer and its color deeper with increasing dis- 

 tance from the rivers, the proportion of carbonates diminishes, 

 and may entirely disappear beforo the border of the loess is 

 reached. The shells and concretions are limited to the calca- 

 reous portions of the loess, or they may be still more restricted 

 in their distribution. 



With increasing distance from the streams goes another 

 change in the composition of the loess. The complex silicates 

 (feldspar, mica, hornblende, augite, etc.), which are found to 

 be very significant ingredients of the formation along the river 

 bluffs, become less and less abundant, as the other normal 

 characteristics disappear. They may be found, however, in the 

 loams remote from the streams, after almost every other true 

 loess feature has disappeared. 



The thickness of the loess diminishes regularly with increas- 

 ing distance from the rivers. So far does this thinning pro- 

 ceed, that at a distance of a few miles from the streams, but a 

 thin mantle remains. 



The features thus indicated as pertaining to, and defining 

 the loess in the region under consideration, are the features 

 which characterize it throughout its distribution to the north 

 along the Mississippi, and along its tributaries in Illinois, 

 Missouri, Iowa, Wisconsin and Minnesota, and along the trib- 

 utaries of the lower Ohio, in Illinois and Indiana. 



Throughout much of this territory the loess lies upon the 

 glacial drift. In southern Illinois, for example, the drift for 

 many miles north of its southern boundary is overspread with 

 loess, or with clay-like loam which may be traced into direct 

 continuity with the normal, open-textured, calcareous, shell- 

 and concretion-bearing loess, along the immediate valleys of 

 the streams. Here too, in scores and hundreds of places, espe- 

 cially in southeastern Illinois, it may be seen that the surface 

 of the drift upon which the loess rests, is one which gives no 

 evidence of exposure to the atmosphere before the mantling 

 loess was spread upon it. Had such exposure found place, the 

 fact would have left its record in the oxidation of the exposed 

 surface, or in the accumulation upon it of an old soil, traces of 

 which would still be found beneath the loess. But in south- 

 eastern Illinois and the adjacent parts of Indiana, no zone of 

 oxidation, and no vegetable layer, or trace of old soil, separates 

 the loess from the till beneath. It would not be necessary to 

 suppose that such a zone as that here referred to would neces- 

 sarily be preserved at all points, until the present time. But 

 its universal absence over large areas, under conditions which 



