Pench — Climatic Features in the Land Surface. 171 



along the upper branches of the Nile the Victoria Nyanza. 

 There are good evidences that the two first named lakes occupy 

 depressions formed by crustal movement. Now we have seen 

 that generally crustal movements, however profound they may 

 be, do not interrupt the formation of slopes of degradation. 

 Why did such interruption occur in central Africa ? In order 

 to give an answer, let us consider how the formation of isolated 

 basins by crustal movement is hindered in the regions of fluvia- 

 tile drainage. A tract of land, sinking down, is filled up by 

 sediment during its subsidence ; thus the rivers maintain their 

 slopes and the formation of closed basins is counteracted. 

 Elevations occurring in river basins are often cut through by 

 rivers during their elevation, and then they will not transform 

 neighboring regions into closed basins. The formation of 

 those basins by crustal movement will occur only when the 

 rivers in the disturbed regions are too feeble to fill up the 

 sinking regions and to cut through the rising chasms. The 

 central African great lakes are located in the neighborhood of 

 insignificant river action. East of them extend the dry pla- 

 teaus of German East 'Africa. Here crustal movement has 

 produced many closed basins called Graben (tectonic troughs) 

 by Suess. Many of these Graben are empty, or filled only at 

 their bottoms with soda-lakes ; others are nearly filled, as for 

 example, Lake Rudolph. A slight climatic change would fill 

 the basin of this lake so that its waters would overfiow in wet 

 years. This is "the case with Lake Tanganyika. Its outlet, the 

 Lukuga, flows only in wet years ; in dry years the great lake 

 is without outlet. A stronger climatic change would trans- 

 form Lake Rudolph into a normal freshwater lake with per- 

 manent outflow. This type of lake is represented in the 

 neighborhood by the Victoria Nyanza, by the Albert and 

 Albert Edward Nyanza, and farther south by Lake Nyassa. 

 We can understand best the existence of the large freshwater 

 lakes of east equatorial Africa by assuming that their water 

 drowns basins formed in a dry climate, such as prevails farther 

 east. It is very significant that these great lakes occur in a 

 region of transition between the dry and the wet tropical 

 climate, a region which would be affected very much by cli- 

 matic changes. The same is true for Lake Baikal in eastern 

 Siberia. It belongs to a zone of great inland lakes at the 

 northern border region of the central Asiatic deserts. Three 

 of these lakes have salt water ; two of them are so large that 

 they are called erroneously seas, namely Lake Caspian and Lake 

 Aral ; the third is the shallow lake Balkash. An increase of 

 precipitation over Russia would cause a raising of the level of 

 Lake Caspian and an extension of its surface, and if on this 

 larger surface evaporation would not balance the increased 



