Page:The Journal of geology (IA journalofgeology21894univers).pdf/346
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 more general and widespread. 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 comparative 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 heterogeneous 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 much 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 westernmost north-and-south drainage valley, that of the Missouri-Mississippi river. Almost everywhere it is heaviest nearest the