300 Vrof. Baron F. RlchiJiofen — On the Origin of the Loess. 



of phenomena, which, although they are almost the reverse of each 

 other in regard to their outward appearance and the conditions they 

 afford to human existence, are closely allied in nature in respect to 

 their geograj^hical distribution. It need only be noticed that the 

 salt steppes of Central Asia are surrounded on all sides by Loess 

 regions. The chief difficulty, when the theory was first advanced, 

 was the want of a sufficient source whence the enormous amounts of 

 dust required by it could have had their origin. But the problem 

 has since been resolved in a most ingenious and, as I believe, satis- 

 factory way by Mr. Eaphael Pumpelly.^ 



There exist, besides the undrained salt steppe, regions of a some- 

 what different kind, which serve as permanent resting-places to the 

 subaerially deposited dust. They are suiEciently distinct from the 

 former class of places of deposition to be styled a second class of 

 these, although, as a matter of course, there must be a series of 

 gradations connecting both. To this second class belong those wide 

 grass-covered plains which are known by the names of prairies, 

 savannas, llanos, pampas, steppes of Southern Eussia and Siberia, 

 etc. They, too, are subjected to the alternation of a dry and a wet 

 season. They are distinguished from the drainless salt steppe by 

 their level surface and by the fact that they are crossed and partly 

 drained by larger rivers, the origin of which lies almost exclusively 

 beyond their boundaries. Some of these regions are very moist 

 in the wet season, and bear a luxuriant vegetation of grass and 

 flowering herbs, but dry up completely during the rest of the year. 

 The rate at which the growth of soil takes place will depend upon 

 the character of the adjoining regions from which the winds prevail- 

 ing in the dry season remove the loose soil. It can no longer be 

 doubted that the " black earth " of Southern Eussia is growing in this 

 way, and I am inclined to the same opinion with regard to the "Eegur " 

 of India. The black colour, which is proper to the uppermost 

 layer only, appears to result solely from the formation of vegetable 

 mould, the deeper portions showing the brown colour of the Loess, 

 together with its structure, although this appears to be less perfect 

 than in the former case. The bones of mammals will probably be 

 badly preserved in this soil, because, in consequence of the ample 

 rains and the slow rate at which the soil grows, they will partly 

 decay before being perfectly covered up. This may. not necessarily 

 apply to those land shells, the animals of which die underground, 

 at their places of refuge. 



Another difference from the steppes of the first class must be 

 produced by the circumstance that the salts will be removed, in 

 part, by the water which percolates the soil and takes its way to 

 the river channels. 



When, after my return to Europe, I commenced to study more 

 closely than I had done in former time the Loess of this continent, 

 its perfect similarity in regard to composition, structure, and mode 

 of occurrence with that of Asia, could not fail to strike me forcibly, 

 and led me irresistibly to the conclusion that it must have been 

 ^ Amer. Journ. of Science and Arts, vol. xvii. 1879, p. 133. 



