298 Pro/". Banon F. Richthofen — On the Origin of the Loess. 



In rainless deserts the wind will gradually remove every particle 

 of iine-grained matter from the soil, though a new supply of this 

 may constantly be provided by the action of sandblast. The sedi- 

 ments of desiccated lakes, the soil which is laid bare by the retiring 

 of the sea, the materials which are carried down by periodical 

 torrents from glaciated regions to desert depressions, the particles 

 which on every free surface of rock are loosened by constant 

 decay — all these will be turned over again and again by the wind, 

 and undergo an incessant sifting, until every earthy grain is blown 

 off and nothing but moving sand and wind-worn pebbles remain. 



The dust may travel great distances, and if the wind during the 

 dry portion of the year blows constantly in one direction, that 

 distance will increase, while the deposition of ajolian sediments will 

 be cumulative in places situated in the same direction. If the dust 

 is deposited on mountain ranges endowed with considerable fall of 

 rain and drainage towards the sea, it will be finally carried to this 

 reservoir. 



There are, however, chiefly two great classes of places where the 

 dust of continents will rest permanently, and continue to accumulate 

 through ages. 



The first are what maybe termed the central regions of continents, 

 that is, those regions where, notwithstanding some rain which chiefly 

 falls in one season of the year, the water has no drainage towards the 

 sea, but is collected in inland basins from which it evaporates. This 

 is the case in the Great Basin of North America, in Persia, and in 

 Central Asia, from the Pamir to the Khingan range, and from the 

 Himalaya to the Altai. The prevailing vegetation, independently of 

 altitude, is that of the salt steppe. Grass and herbs take hold of the 

 dust, whilst the debris tliat collects slowly on the hill-sides is, by 

 very slow gradations, washed down the slopes by occasional rains, and 

 will, if there happens to occur a period of heavier rainfall, be spread 

 over a portion of the surface of the steppe, giving rise again to the 

 growth of vegetation, which in its turn takes hold of the falling 

 dust. In this way the dust will accumulate slowly but constantly 

 through ages on those portions of the surface which are covered by 

 vegetation. In the course of time it may reach a thickness of 

 hundreds and perhaps of thousands of feet. The salts resulting 

 from the decomposition of the rocks, and carried partly through the 

 air together with the dust, and partly by water down the hill-sides, 

 will remain in the soil, and collect chiefly in salt pools situated in 

 the lowest portions of each basin, where, at the same time, stratified 

 soil is deposited. As the surface which bears the vegetation is by 

 slow degrees rising to a higher level, the tubes in the soil which 

 contained the roots of former generations will retain their shape. 

 The land-shells which feed on the steppe and withdraw to some 

 depth underneath the surface in seasons of drought or cold will be 

 entombed where they die, and the most delicate shells will be pre- 

 served. The same will be the case with tlie bones of mammals and 

 birds living on the steppe, the dryness of the climate preventing 

 the decay of any organic matter, as well as the formation of vegetable 



