128 On the Tchonioi Zem, 



any adequate cause is brou^rht into play. The opening or fissuring 

 of these masses, then, is due, in the first place, to an extreme 

 climate, which subjects the surface to intense and long droughts, 

 alternating with heavy debacles, arising from the melting of thick 

 coverings of snow and ice. During the hot and parching summers 

 the argillaceous grounds necessarily split into rents, and wherever 

 these occur upon slopes, the thaw of the succeeding spring libe- 

 rating vast bodies of snow and water, the smallest crack of the 

 previous year is enlarged mto a gulley, which, widening as it 

 descends, becomes in a few seasons a broad and deep ravine, 

 through which the masses of melted snow, mud, sand, and clay 

 are transported into the adjacent river. It is the conjunction, 

 therefore, of the very incoherent nature of the upper deposits of 

 Russia with the extremes of her climate which explains the form- 

 ation of the innumerable ravines that fissure her surface. It 

 would indeed be a curious problem to ascertain to what extent 

 these ravines encroach annually upon the best arable and pasture 

 grounds of the empire, and in what progression this waste 

 takes place; as proved by the rapidly increasing deltas at the 

 mouths of the Volga, Don, &c., and by the very perceptible 

 silting up of the Sea of Azof. In no instance have I seen any 

 means adopted to check this continual wear and tear, by which 

 millions of tons of the richest soils are annually destroyed, and 

 transported away by the great rivers. In the mean time, the 

 geologist has to thank these *' avrachs " for most of his best sec- 

 tions, for it is generally near their mouths, where the denudation 

 has been deepest, that the parent- rock or true subsoil is laid bare. 

 I may here also state, that it is owing to the ravined nature of the 

 sides of the hills, and the wide mouths of these gullies, that the 

 great roads of Russia pass almost invariably over the highest 

 table-lands, where the "avrachs" are, comparatively speaking, 

 absent. Instead of travelling along the banks of the great water- 

 courses, as we might think would be the case by an inspection of 

 a physical map, it has been found impossible to maintain roads 

 along these lower levels, — first, from their being inundated during 

 certain seasons ; and, secondly, by the innumerable mouths of 

 the ravines, which defy all the efforts of bridge-makers, aud are 

 for ever changing their courses and dimensions. 



Returning, however, from this allusion to a phenomenon which 

 affects all the incoherent deposits of Russia, to our special subject, 

 it must be clearly understood, that the black soil of which we are 

 treating does not, by any means, occupy all the vast country in 

 question. It occurs, indeed, in areas sometimes consisting of 

 sev^eral large parishes, and is invariably the superior deposit, 

 covering all other accumulations of clay, sand, 6cc. In thickness 

 it \anes from a few feet to 15 or 1:0 feet. In travelling over 



