GEOLOGICAL IMPORTANCE OF SEDIMENTATION 329 



tides and storms, a delta is rapidly developed, a considerable portion 

 of which is a land surface reclaimed by the river from the sea. 



GEOLOGICAL IMPORTANCE OF DESERT DEPOSITS 



To consider the areas occupied by each of the above divisions, it 

 is to be noted that the arid regions at present cover about 11,500,000 

 square miles; that is to say, at the present time over one-fifth cf the 

 land of the world has no outlet for drainage to the sea/ Within these 

 regions extensive sedimentation goes forward, the waste of the moun- 

 tains filling interior basins, either by wash from the mountain slopes, 

 by streams which sink within their subaerial deltas, or which may 

 flow into shallow interior seas. By far the greater portion cf the 

 waste is laid down by rivers, owing to the vanishing of the water into 

 the dry air and the porous soil. At such places the streams flow in 

 channels, but not in valleys, and in time of flood spread in a thin 

 sheet of water for miles over the desert plains. Instances of this 

 nature has been noted by Davis in Turkestan,^ and by McGee in 

 the Sonoran desert. ^ In Australia large temporary lakes are formed 

 during the wet season, which during the seasons of drought become 

 arid and burning deserts. '^ At irregular intervals, sometimes extend- 

 ing over several years, the most arid portions of the interior will for 

 a few days assume the appearance of a boundless, though shallow, 

 inland sea.^ The conditions are thus.of rather widespread occurrence 

 in desert regions for the formation of stream deposits, current-marks, 

 and mud-cracks associated with river flood-plains and broad, level, 

 sandy tracts and playas — features possessed in common with deltas 

 of arid climates and the mud-flats of the littoral. 



In topographic youth torrential deposits near the mountains, and 

 finer alluvium in the central portions, may accumulate tc great depths 

 in the interior basins. In maturity the waste is more widespread, 

 though over much of the region more shallow in depth, while in old 



1 Dr. John Murray, "Origin and Character of the Sahara," Science, Vol. XVI 

 (1890), p. 106. 



2 Explorations in Turkestan, p. 54 (Monograph, Carnegie Institution, 1905). 



3 "Sheet Flood Erosion," Bulletin 0} the Geological Society of America, Vol. VIII 

 (1897), p. 87. 



■ 4 D. ^. Petherick, Mill's International Geography, p. 615. 

 5 C. /H. -Barton, Mill's International Geography, p. 580. 



