706 C. R. KEYES^ MID-COINTINENTAL EOLATION 



ness of several hundreds of feet, frequently appear on the surface to be 

 gravel beds. 



The original determination of the supposed lacustrine origin of the 

 Great Plains deposits rested mainly on the assumption that the prevail- 

 ingly silty texture could be produced on such an extensive scale only 

 under conditions presented by quiet lake waters. In the north, where 

 the great through-flowing Missouri Elver has been able to degrade rather 

 than aggrade its course, and where on each side the substructure is more 

 or less deeply and sharply trenched into the "bad-lands," the marly silts, 

 as they are commonly called, are ashen in color, remarkably uniform in 

 texture over wide areas and throughout great thicknesses. Farther south, 

 where the country is not cut into by rivers, deep-well borings and such 

 outcrops as occur indicate that the deposits are still quite similar in 

 lithologic character to the more extensively exposed mauvaises terres. 

 All characteristic features continue southward and westward around the 

 southern end of the Eocky Mountains into central New Mexico until, as 

 shown farther on, the deposits merge completely into the undoubted 

 desert soils and appear to be in every way identical with them. 



On the other hand, in its relief features, its texture, its general appear- 

 ance, and its stratigraphic characteristics the "lake deposits" of the Great 

 Plains strikingly resemble the loess. This fact in itself is of great sig- 

 nificance. 



SIMILARITY OF PLAINS SOILS TO LOESS 



Had the loess deposits of the Mississippi Valley never been called such, 

 and had they never been compared with the deposits of the Ehine, it is 

 probable that their genetic relations with the Plains soils would have 

 been long since surmised. Early geologic explorers of the upper Missis- 

 sippi region designated the loess as silicious marl. Between the loess 

 and the Plains marls color alone seems to be the distinguishing feature. 

 The genetic relations also of the ashen marls of the Plains and the thick 

 black loams of the prairies to the eastward present many suggestive 

 phases of great interest. They have never been critically investigated; 

 yet there seems to be here a fertile field of inquiry. That the two may be 

 the same geologic product appearing under somewhat different physical 

 conditions is not at all improbable. 



When, a decade and a half ago, I first set forth®^ the evidences in sup- 

 port of an eolian origin of the loess deposits lying along the Missouri 

 Eiver, I was inclined to derive all of the loess materials directly from the 



American Jour. Sci. (4), vol. vi, 1898, p. 299. 



