ALLUVIAL DEPOSITS 313 



freshet deposits, is clothed with luxuriant forests and dense 

 tangles of undergrowth, or with brakes of cane, or with sub- 

 tropical shrubbery, only a few of the broader inter-stream tracts 

 being grassed. Partly by reason of this mantle of vegetation, 

 the current of each overflow is checked as the river rises above 

 its banks, and most of the sediment is dropped near by ; and so 

 the Mississippi, the White, the Arkansas, and the Red, as well 

 as each lesser tributary and each distributary from the great 

 Atchafalaya down, are flanked by natural levees of height and 

 breadth proportionate to the depth and breadth of the stream. 

 The network of waterways is thus a network of double ridges 

 with channels between; and each inter-stream area is virtually 

 a shallow, dish-like pond in which the waters of the floods lie 

 long, to be drained finally, perhaps, through fresh-made breaks 

 in the natural dikes, weeks after the stream flood subsides. In 

 the southern part of the district the inter-stream basins approach 

 tide level and drain still more slowly; in the sub-coastal zone 

 many of the basins are permanent tidal marshes. In the western 

 part of the district is an area in which the inter-stream basins 

 lie so high that they are invaded only by the highest floods and 

 veneered with only the finest sediments; in some cases these 

 sediments are so fine and so compactly aggregated and the 

 surface is so ill drained and watered that trees may hardly 

 take root, and these are either drowned by the floods or with- 

 ered by the sun in the drought. Such portions of the sur- 

 face are but scantily covered with coarse grass and form 

 the ''black prairies'' of southern Arkansas and northwestern 

 Louisiana. 



It is to similar processes as those described that the Nile valley 

 owes its remarkable fertility. The sediments deposited over the 

 plains during the season of freshets consist of fine sand brought 

 down by the Blue Nile and the Atbara from the decomposing 

 siliceous rocks of mountainous Abyssinia. The gneisses and gran- 

 ites yield their detritus to the lixiviating influence of the moun- 

 tain torrents and majestic Nile, the clayey particles being borne 

 seaward, while the fresh quartzose, f eldspathic and other siliceous 

 particles, and smaller traces of apatite and alkaline carbonates 

 remain in just the right stage of subdivision to yield a soil, 

 which has brought forth for a period of over 4000 years crop 

 after crop without artificial fertilization. 



The following table will serve to show the physical cnaracter- 



