Dr. Hume — Notes on Russian Geology — The Black Earth. 349 



Glacial epochs, without assuming that there was either any far 

 re-advance of the ice-border or any epeirogenic movements attending 

 the glacial retreat of such magnitude as to induce the fluctuations of 

 which the forest beds and marginal moraines bear witness. Though 

 the whole history of the growth, culmination, and departure of the 

 ice-sheets is indeed very complex and long, as measured by our 

 familiar historical time units, it was yet, in my opinion, geologically 

 very brief if compared with all preceding geologic periods and 

 epochs. Instead of being subdivided by long intervals of temperate 

 or warm climate, the Ice-age seems to me to have been essentially 

 continuous and single, with important fluctuations, but not of 

 epochal significance, both during its advance and decline. 



III. — Notes on Eussian Geology. 



By W. F. Hume, D.Sc, A.E.S.M., F.G.S. ; 

 Demonstrator in Geology, Royal College of Science. 



{Continued from the July Number, p. 312.) 

 III. The Black Earth. 



Prof. Krasnov states that the decomposition of the roots of plants 

 gives rise to the humus, but attaches much more importance to the 

 effect of climatic conditions. Thus, he remarks, " that it is im- 

 possible not to note that the Tchernozem occupies a band stretching 

 from S.W. to N.E., that is, the position where the influence of a 

 temperate and moist climate ends, and the region of dry S.E. winds 

 begins. In the N. and N.W. of Eussia the woods predominate, 

 unless destroyed by man, or prevented by the marshy nature of the 

 country. Where Black Earth begins, the woods end, and little by 

 little steppe assumes sway, first as half steppe (that is to say, the 

 alternation of wood and broad black earth plains), and finally to 

 the S. as true steppe, where the view is, in most cases, unimpeded 

 even by a single bush." 



Prof. DokoutchaiefP has recently discussed the whole of this 

 question ("The Eussian Steppes " : a pamphlet written for the World's 

 Columbian Exhibition at Chicago, 1893). He shows that in the 

 N. of the Poltava government the outlines of the steppe and forest 

 boundaries are clearly marked out, and that it is yet possible to trace 

 the positions formerly occupied by extensive forests, by means of 

 the special character of the soil to which they have given rise. 



He has also shown that there is a transitional structure in the 

 Black Earth itself. In some of the Eastern governments (Tambov, 

 Penza, Simbirsk, and Samara in part) a maximum humus percentage 

 of 12 per cent, is attained, whilst to the N.W. and S.E. of these the 

 quantity of humus diminishes. 



Accordingly he concludes that to the N.W. the superabundance 

 of moisture, and to the S.E. the dryness, prevent the formation of 

 humus, and that it is in the ti-ansitional climatic band that the most 

 favourable conditions for its production exist. I am not aware that 

 any chemical research has been carried out on Black Earth soils 



