The term Rendzina is applied to soils derived from the weather ing of calcareous rocks.
There are vast regions that are intercalated among the forests covered by bog, especially in the west. Bog soils are formed where the pores of the soil are filled with moisture for a prolonged period, in regions of slight evaporation and marked at mospheric precipitation. The organic matter, paludinous herba ceous growths (Carex, etc.) and paludinous moss (Sphagnum, etc.) and sometimes the remains of alder, dwarf-birch, etc., instead of decomposing into humus, becomes enriched with carbon and trans formed into peat. Micro-organisms play a large part in this car bonisation process, which penetrates to varying depths and which, where moisture rich in oxygen exists in deeper layers, may be superimposed on a humus layer. As a result of evaporation on the surface of the swamped soil, iron is precipitated in the upper soil horizon directly beneath the peaty layer in the form of nodules of bog-iron ore. Bog-iron ore has been worked in a primitive way from very early times in Russia. The swamps and marshes have played a twofold part in history; they hindered the development of communications, but they frequently formed retreats for the Slays and the Finns in early times. Novgorod escaped destruc tion by the Tatar horde in the 13th century because the Tatars could not cross the marsh in the rainy summer, and the strategic importance of the Pripet marshes has been evident from early days. Today the peat bogs have great commercial importance, for Russian scientists have invented a method of using peat fuel in the production of electricity.
The forest and marsh zone of European Russia is of peculiar interest historically, for it was in this less attractive region that the Slays settled after the devastation of the fertile steppe by nomad Turkic hordes, and it was in this region that the Great Russian race consolidated the kingdom of Moscow, which after wards re-absorbed the steppe. Here, too, various small minorities, Finnish and Turkic, were able to retain their national individuality even to the west of the Urals, and are now separate cultural units within the Russian state. The forests of this belt are subject to two forms of climatic zoning, a north to south zone related to temperature and a west to east zone related to decreasing humidity.
The beech just makes an appearance in the Podolian bend of the Ukraine, but penetrates no further east, except in the moister re gions of the Crimea and the Caucasus. The oak approaches, but does not cross, the Urals, while the Siberian pine, larch and cedar penetrate from Asiatic Russia into the extreme north-east of European Russia.
Concerning north to south distributions, one notes that the oak appears south of a line drawn through Pskov, Kostroma, Kazan and Ufa, though islands of oak penetrate further to the north. Profitable production of apple, pear and cherry is roughly coinci dent with the oak belt. Hornbeam does not penetrate much beyond the northern boundary of the Ukraine, though maple and ash accompany the oak almost to its northern limit. The lime tree combines the two zones, preferring the east and not as a rule ex tending further north than lat. 62°-64° N. Apricots and walnuts, on the other hand, prefer the west, reaching 50° N. lat. in eastern Ukraine and spreading further north to the west.
The Siberian coniferous zone is characterised by its wider north to south extent, by the great fringe of stunted birch to the north, where trees a hundred years old may be but a few feet high and are thickly encrusted with lichens, and by certain distinct varieties of conifers, e.g., the silver fir, stone pine and the Siberian larch. In the virgin forests of the valleys and lowlands of Western Siberia are larches, pines and silver firs, mingled with birch and aspen, while poplar and willow fringe the streams. In this region are the urmans, dense thickets of trees often rising from a treacherous carpet of thickly interlaced grasses, concealing deep marshes, impassable except in winter. In the south-eastern trans-Baikal region, under the influence of the drought of the Gobi steppe, the forest thins out. The
tion of spruce and Scotch pine, the chief trees of the coniferous forest, depends largely on the parent rock. Spruce grows best on clayey, but not boggy, soil and is therefore closely related to the distribution of glacial boulder clay, while Scotch pine spreads best on sand and peat.