The Black Earth. 309 



The amount of silica is always very noticeable. The high per- 

 centage is due to the large number of polished sand-grains always 

 present in samples of Tchernozem. These, however, vary greatly in 

 size, from the grains iV of an inch in diameter already mentioned 

 from Bielgorod, down to particles almost invisible to the naked eye. 

 The variations appear in some respects, to follow the rule, that the 

 further south the deposit the smaller the sand-grains contained in it. 

 It would require a close examination of a large number of samples 

 before this could be established as an absolutely proved fact. 



In doing so, particular care must be taken to select the specimen 

 from an uncultivated spot. This is now a somewhat difficult task, 

 so much of the steppe-land having been brought under cultivation. 

 Examination and enquiry have both, however, led to .the same 

 result — the deeper the earth from the surface the more sand-grains 

 does it contain. Both in character, position, chemical analysis, and 

 history, the Black Earth is thus intimately connected with the Loess, 

 and is due to special circumstances affecting the latter. 



A historical review of the whole subject is of particular interest, 

 as showing the various possibilities which have been brought forward 

 to account for these soils, the treeless prairies and steppes, and the 

 sharp distinctions between present geographical conditions in N. 

 and S. Russia. At the same time a remarkable parallelism is to be 

 observed between the theories propounded in America, and those 

 which have from time to time been enunciated by Russian geologists. 

 In the Journ. Poltava Agricult. Soc. February-May, 1891, Prof. 

 Krasnov has given an admirable summary of the latest position of 

 these discussions as viewed from Russian standpoints. Many of his 

 conclusions have struck me as being of such general interest that it 

 may be advisable to include them in the present discussion, especially 

 as they are at present practically buried in a local Russian journal. 

 Pallas was one of the first to consider the origin of the Tchernozem. 

 He, in common with later observers, regarded it as having been 

 formed on the bed of a gradually-dying sea, the water-plants and 

 other organisms of which, rotting in contact with the atmosphere^, 

 formed its mass, and gave rise to its richness in nitrogeneous products. 



Murchison {loc. cit.) considered it as not having beea formed in 

 situ, but derived from the disintegration of the Black Jurassic shale, 

 which is found in the more northern districts of the empire. He 

 held that powerful currents had acted as the transporting agents, 

 though no reason was assigned for the production of this rush of 

 tumultuous waters. At the present day ice-action explains all these 

 varied results in a more reasonable manner. 



Ruprecht seems to have gone half-way in regarding the Black 

 Earth as of grassy origin, though he still held the view that the 

 boulders strewing the northern parts of the country had been 

 brought thither by ice-bergs. 



But, with the acceptance of the theory propounded by Nikitin, 

 Krapotkine, and Inostranzeff, that a great fresh-water sea did not 

 cover N. Russia, but rather that a huge ice-sheet spread over the 

 face of the country, a new aspect was obviously given to the dis- 

 cussion, and it was sought to explain the phenomena by other means. 



