Russia

The basin of the Petchora is divided from the land farther west by a minor fold belt bringing up Devonian and some crystalline schist along the line of the Timan hills that may be traceable as a branching of the mid-Urals northwestward to Cape Kanin. The whole region has a very low relief and is consequently un protected from Arctic cold. Apart from the Timan hills this region is below the 600 foot contour. Its lakes are discussed in the article on Europe (q.v.). Its chief rivers are the Petchora for which Strelbitsky gives a length of 915 m. and Tillo a length of 1,024 m., the Mezen for which the estimates are 496 and 507 m., and the Northern Dwina for which these authorities give a length of 358 and 447 m. respectively. These determinations vary because Strelbitsky ignored minor windings. The upper waters of Petchora and Northern Dwina interlace with those of the feeders of the Kama, a tributary of the Volga. As a conse quence of the low relief the Petchora is navigable for 77o m., the Mezen for 450 m., and the Northern Dwina for 33o m., while the Vychegda, a large tributary of this river, is navigable for 500 miles. The Onega river has rapids on it.

The Urals.

The structural relationships of the Urals have been mentioned above. Here the Palaeozoic strata are folded on the west of a longitudinal axis which exhibits crystalline rocks and is faulted on a large scale. The Devonian rocks in this area are of marine origin. Its greatest heights are chiefly in the regions of folded Palaeozoic rocks and the folds die away into the Rus sian platform westwards in small parallel chains known as Parma. Though there is much copper in the Permian rocks on the west the main metalliferous veins of the area are in the faulted zone of crystalline rocks to the east. The greatest height reached in the Urals is 5.535 ft. in the northern Urals (at Tel-pos-iz or Mural-Chakhl). The passes are often low, that on the way from Perm to Sverdlovsk being only 1,245 ft. above sea-level. The range is continued to the Arctic coast, Vaygach Island, and Novaya Zemlya. Details are given under articles Ural Mts., Uralsk Area, Novaya Zemlya, etc. On the south the Urals finger out in a plateau region dissected by feeders of the Ural, and a few feeders of the Volga. The Ural river drains the southern Ural mountains, turns west and then south to the Caspian Sea which it reaches after a course of 1,477 m.

The Ob Basin.—Eastwards the Urals fall rapidly down to the Ob basin which is one of the largest areas of unbroken lowland on the earth. The floor material is almost entirely alluvial with a little Tertiary here and there. It rises in the westward prolonga tions of the Sayan mountains and is 2,26o m. long but its very large tributary the Irtish comes from the Dzungarian Gate and adds enormously to the area of the basin, which is bounded south wards by the Turgai and the hills of Semipalatinsk, while the Tobol, another large tributary flows to the Ob from the north of the Turgai. It is widely thought that under the alluvial floor would be found a continuation of the platform of North Euro pean Russia let down through the dislocation of the east border of the Urals. The lowland extends without an appreciable break

to the Yenisei, and only at some places near that river do the older rocks help to form the surface.

The Yenisei Area.—East of the Yenisei the character of the land changes abruptly, a sharp edge rises rapidly to more than 600, and in one place to quite 3,30o ft. above sea-level and east of this edge is a great dissected plateau of ancient rocks. Archaean rocks with granites, etc., are exposed near the Yenisei and away to the north-east, but there are larger areas of Cambrian and Silurian rocks and these are covered over a vast stretch of coun try by Permo-Carboniferous rocks. The plateau extends east wards to the Lena along the valley of which is found evidence of an invasion of the sea in Cretaceous times. The plateau is an ancient block to which Suess gave the name of Angara land.

Large areas on this plateau rise somewhat above the 1,6o0 ft. contour, and the diversities of surface are due largely to river dis section. There is evidence in the Taimyr peninsula of a fold axis with a general direction west-south-west to east-north-east but apparently no heights reaching above the 1,500 ft. contour.

The Yenisei is the collecting stream beneath the western edge of this old block and it rises in the Sayan mountains receiving nearly all its tributaries from the eastern side. Of these the upper Tunguska or Angara comes from Lake Baikal, the middle or stony Tunguska, and the lower Tunguska from the block itself.

The river is some 3,00o m. long. Like the Ob, this river may be said to be formed of a pair of streams, the upper Yenisei and the Angara.

Eastern Siberia.—Beyond the Angara block eastwards stands an important series of mountain-ranges concerning which much new knowledge has recently accrued (see S. Obruchev, Geogr. Journal, lxx., 1927, p. 464), though it is not yet available for maps. The Stanovoi (Jurgur) mountains and the Kolimski (Kolyma) mountains are more or less parallel to the coasts of the sea of Okhotsk but the former are also roughly parallel to the edge of the Angara block on the other side of the Lena, and Ioo m. or so north of Okhotsk they are linked not only with the Kolimski mountains to the east but with the Verkhoyansk moun tains which go westwards and northwards keeping more or less parallel to the edge of the Angara block with the Lena between them and it. Like the other great rivers mentioned above, this one is formed of a pair, the upper Lena and the Vitim. The length of the Lena is estimated at 2,86o miles. The Verkhoyansk and the Kolimski mountains thus form a great semi-circle of moun tains apparently determined in the west by the Angara block and in the east by the depression of the sea of Okhotsk. Within this semi-circle most atlases show ranges stretching northward, but according to Obruchev the older surveys are very incomplete. He finds that the ranges are disposed in more or less parallel curves within the main outer one above mentioned.