Prof. Baron F. Richthofen — On the Origin of the Loess. 299 



mould, which would be created in a moist climate through the decay 

 of the organic matter. 



In this way the deepest valleys, the wildest gorges, and the 

 largest depressions in undrained regions may be gradually filled up 

 with the deposits of dust, interchanging near the encasing slopes 

 with the angular debris of rocks, but increasing in homogeneousness 

 of composition and structure, and in freedom from any foreign 

 ingredients towards the central portions of each basin. The 

 inequalities of the ground will disappear, the lower hills will be 

 buried, and the surface of the steppe will have a trough-like shape 

 between every two protruding rocky ranges. If then, in con- 

 sequence of a lasting change of climate, such a basin should 

 gradually be filled with water, and an outward drainage be opened, 

 erosion would soon furrow deep channels through the earthy deposit 

 and expose its interior structure ; the fine tubes marking the site 

 of the roots of countless generations of plants, the remains of the 

 shells that had fed on the grass, and the bones of the mammals that 

 have lived on the steppe would become visible ; and the earth so 

 exposed would be what is called Loess. 



Such was the line of argument which I founded on the study ofi 

 the Chinese Loess. I concluded that the same regions where the 

 traveller of the present day moves between stupendous walls of 

 yellow earth, and gazes with daily renewed wonder at the fantastic 

 shape of rocks of earth produced by erosion ; where millions of 

 people live in caves dug in the vertical faces of the Loess, while 

 on its terraced surface they cultivate fields whicb are highly pro- 

 ductive in wet summers, and terribly barren when moisture is not 

 supplied in sufficient quantity — that these same regions, through 

 which the Yellow Riv6r and its tributaries now take their courses, 

 were once covered with, dreary steppes only fit for nomadic life, and 

 had no drainage towards the sea. 



I had soon an opportunity of verifying the theory by a visit to 

 Mongolia, where I saw precisely what my experience in the Loess 

 regions had caused me to expect, namely, the very same shape of 

 surface which I had observed in these, a steppe vegetation growing 

 upon an impalpably fine earth mixed with grains of sand, and 

 accumulations of the debris of rocks at the foot of the hill-sides. 

 But in no place could I see the inner structui-e of the soil exposed 

 to view. Proceeding, however, to the boundaries of the undrained 

 region, where the drainage of some marginal basins had begun, but 

 the channels of erosion were still shallow, the first sure signs of 

 true Loess made their appeai'ance on the side of every natural cut 

 in the ground. From this first stage of the conversion of steppe 

 basins into Loess basins, all grades of passage to the wildest and 

 most grotesque landscapes, where the Loess was exposed to view in 

 a thickness of a thousand feet, could be observed in rapid succession. 



It appears to me that the theory answers all requirements as 

 regards the Chinese Loess, in so far as it easily explains all its 

 properties and every incident in the mode of its occurrence. It 

 combines, moreover, into one class of natural processes two kinds 



