SIBERIAN HIGHLANDS 63 



highlands of moderate elevation, which includes the 

 Altai and Sayan chains and the Trans-Baikal highlands 

 with the Vitim and Aldan plateaus, separates Mongolia 

 and Amuria from eastern Siberia. 



As may be expected from its continental situation, it 

 is a region of severe climatic extremes and reduced 

 atmospheric moisture. The rainfall derived from the 

 northern and eastern quarters occurs chiefly in summer, 

 but there is a heavy snowfall in winter. The moderate 

 height of the mountains, sufficient to condense a large 

 proportion of the vapour of the winds, allows some of 

 it to penetrate further south than would such a lofty 

 barrier as the Kuenlun. 



As each successive ridge athwart the southward track 

 of the winds levies its toll of moisture, the lower limits 

 of rains and clouds are gradually raised from ridge to 

 ridge, with the result that the southernmost valleys 

 remain dry. Correspondingly, the levels of fairly 

 luxuriant vegetation recede farther and farther up, 

 going towards Mongolia ; and for the same reason, the 

 southern slopes are drier than those facing north. Thus, 

 whilst the Siberian slopes are abundantly wooded and 

 linked uninterruptedly with the Siberian tai'ga, the 

 Mongolian valleys display their forest belts only on their 

 upper slopes. 



Conifers are the only trees that can constitute forests 

 among the Siberian highlands under such extreme con- 

 ditions : birches and aspen, which accompany the rivers, 

 are of little importance as forest-trees, and mostly to be 

 found at lower levels, though on the sunny southern 

 slopes of Trans-Baikalia the birch may reach an eleva- 

 tion of 4,600 feet. Between the Siberian tai'ga and the 

 highland forests there is this difference, that the latter 

 do not exhibit the stunted, even dwarf aspect, which 



