274 GEOLOGY. 



The exigencies of his theory require that mountain chains should 

 cut off the moisture from a contiguous, elevated, undrained region. 

 The dessication of such a region exposed to dry, cold winds fur- 

 nished the dust-like materials that filled up iower lands and became 

 the Loess of this period. Prof. Pompally, contrary to his former 

 views, now advocates this theory.* Clarence King now also lends to 

 this theory a qualified assent. These eminent men would account 

 for the Loess of Nebraska in the same way. 



I admit that some facts concerning the Loess of Nebraska could 

 be explained by this theory. One of these is the wind structure of 

 some of the Loess hills on the Logan, Elkhorn, Loup and Repub- 

 lican rivers. This structure is often found there as distinct as 

 among the shifting sands of our sea coast. In every case, however, 

 where I examined this structure in the Loess I found it to be su- 

 perficial. Out of nineteen such hills none of them possessed this 

 structure over ten feet deep, and few of them over five feet, and 

 many of them only from two to three feet deep. In the deep can- 

 yons where the Loess is exposed vertically from fifteen to one 

 hundred feet I have never found this wind structure over ten feet 

 deep. It occurs, therefore, only in the Loess that has been recently 

 modified by the winds, and long after it was first deposited. 



Another fact which the theory of a subaerial origin would ex- 

 plain, is that the terraces in the valley of Oak Creek and Little 

 Salt are formed of Loess, but the high plateau or divides between 

 these streams are Drift. There are other similar cases in the State, 

 where the Loess is comparatively thin. It is natural to suppose 

 that if the Loess had been a subaqueous deposit, it would have 

 been laid down on the uplands as well as in the valleys — if formed 

 subcerially, the valleys would have been filled up first. In other 

 sections, however, the Loess covers with equal thickness uplands 

 and the flanks of the valleys. West of Crete, as far as the Loess 

 extends, it was probably laid down alike on hills and valleys, with 

 only a few unimportant exceptions. In Dakota and Dixon coun- 

 ties, in southern Cedar, and many counties west of these, the Loess 

 frequently is as thick on the high hills as in the terraced valleys. 

 The isolated uplands now devoid of Loess, on the theory of its 

 subaqueous origin, must have been islands in this old Nebraska 

 like, or else it has been removed by erosion. There are some facts 

 that point to the former theory — the island origin of these spots de- 



*Soe American Journal of Scl.nce and Arts for January, 1879. 



