Of these rivers the double river, Ob-Irtisch, with its numerous affluents, occupies an area of more than 60,000 geographical square miles, the Yenisej-Angara, not quite 50,000, and the Lena, somewhat over 40,000.[209] As the map of the river system of Siberia, which accompanies this work, shows, but a small part of these enormous territories lies north of the Arctic Circle, and only very inconsiderable portions of it are occupied by woodless _tundra_, which is explained by the fact that the greater part of the coast-land bordering on the Arctic Ocean is drained by small rivers of its own, and therefore cannot be considered to belong to the river territories now in question. If we draw the northern boundary of the land that may be cultivated with advantage at 60 deg. N.L., there remains a cultivable area of 90,000 geographical square miles. Perhaps a third part of this is occupied by rocky country which is wooded, and probably capable of being cultivated only with considerable difficulty, but the rest consists for the most part of easily cultivated grassy plains, with little wood, and covered with the most luxuriant vegetation. The soil, in many places resembling the black earth or _tscherno-sem_ of Russia, recompenses with abundant harvests even the slightest labour of cultivation. Notwithstanding this, these regions now support only an exceedingly sparse population, but many, many millions may without difficulty find their subsistence there when once cultivation has developed the rich natural resources of the country.