330 THE JOURNAL OF GEOLOGY. 



across a mountain range, provided there be a favorable current 

 in the upper part of the atmosphere. 



While the conditions requisite for much aerial erosion are 

 limited to rather small areas on the land of the globe, there can 

 be little doubt that deposition is much rhore general and wide- 

 spread. For dust is carried everywhere. And if it be conceded 

 that the atmosphere is never entirely free from dust, it follows 

 that sedimentation occurs wherever and whenever there is a com- 

 parative calm. In places in the ocean, where sedimentation is 

 known to be very slow, atmospheric dust may be supposed to 

 form an appreciable part of the deposits. 



The areas of deposition being much greater than the areas of 

 erosion, it is evident the accumulations of atmospheric sediments 

 as a rule are insignificant, only exceptionally exceeding on the 

 land the secular erosion by water, and therefore accumulating 

 only in such exceptional cases. 



From a dynamical point of view the wind-theory would 

 appear to furnish an adequate explanation of the occurrence of 

 the loess in the Mississippi valley, at least as to most of its 

 phases. The recent denudation of the western plains, of the 

 bad lands, and of the Cordilleran plateau is extensive enough to 

 furnish the materials many times over. The different rocks in 

 these regions and the changeability of the atmospheric currents 

 would combine to bring together and thoroughly mix a variety 

 of materials, like those of which the loess is composed. The 

 winds would naturally distribute over wide areas the heterogene- 

 ous but uniform mixture thus produced. When not taken close 

 to exposures of other materials ninety-nine per cent, by weight, 

 of the loess is composed of particles below the size of .1 mm. 

 and it contains only a small proportion of the finest materials 

 common in clays and residuary earths, just as must be the case 

 in an atmospheric sediment. In the United States, lying in the 

 zone of westerly winds, we find the loess in the continental basin 

 east of the arid regions. It is best developed along the western- 

 most north-and-south drainage valley, that of the Missouri-Mis- 

 sissippi river. Almost everywhere it is heaviest nearest the. 



