136 G. F. WRIGHT — LOESS IN NORTHERN CHINA AND CENTRAL ASIA 



an instability of the earth's crust extending down to a comparatively 

 recent time. 



1. As is well known, the Himalaya, Thian-Shan, Ala Tau, and Altai 

 mountains all bear on their flanks strata of middle Tertiary age, now 

 elevated to a height of more than 10,000 feet. 



2. The existence of lake Baikal. is an indubitable witness to the 

 recency of some of these extensive changes. This lake, 400 miles long 

 and with an average width of 30 miles, occupies a depression on the 

 western side of the Vitim plateau, and lies directly athwart the drainage 

 basin of the Selenga river. The lake is surrounded on every side 

 (except where the Angara river issues from it) by mountains rising from 

 2,000 to 5,000 feet above its surface. Its elevation above the sea is 1,580 

 feet, but near its southern end its depth is 4,180 feet. The drainage 

 basin of the Selenga river, whose sediment is swept into the south end 

 of the' lake, is fully 200,000 square miles in extent, the most of which is 

 more than 3,000 feet above the surface of the lake, giving a very steep- 

 gradient for the streams. The main river, with all its tributaries, now 

 occupies deep trenches of erosion in granitic and archean rocks. These 

 trenches are in their lower courses 2 or 3 miles in width and a thousand 

 or more feet in depth, representing an immense period of time. But 

 nearly all of them contain sedimentary strata at the bottom, sometimes 

 100 feet or more in thickness, of Tertiary age. 



A brief calculation only is required to show that the formation of that 

 portion of the lake basin which receives the sediment of the Selenga 

 river, belongs to very recent geological time ; for, at the most moderate 

 rate at which it can be supposed that the erosion in this valley is pro- 

 ceeding, the whole southern half of lake Baikal would be filled with 

 sediment in less than 500,000 years, whereas it is certainly not one- 

 quarter full. Briefly stated, the calculation is as follows : Estimating 

 the length of that part of the basin receiving the sediment to be 200 

 miles, the breadth 40 miles, and the average original depth one-half 

 mile, all of which figures are in excess of the reality, the basin to be 

 filled would contain 4,000 cubic miles. At the rate with which erosion 

 is proceeding in the Mississippi valley, namely, the removal of one foot 

 from the entire surface in 5,000 years, 40 cubic miles of sediment would 

 be brought into the lake by the Selenga river in that period of time ; so 

 that in 500,000 years the whole work of filling it would be accomplished. 



The estimate concerning the extent to which the sedimentation has 

 already proceeded is based somewhat on the size of the delta at the 

 mouth of the Selenga river. This approximately has a base of 30 miles 

 along the shore of the lake, with an average width of less than 10 miles, 

 while over much of this area the thickness of the deposit is evidently 



