ALLUVIAL DEPOSITS 



309 



when treating of the action of running water, a few figures 

 were given relative to the amount of transported debris de- 

 posited yearly in the Gulf of Mexico In a similar way the 

 amount of debris carried annually to the ocean by some of the 

 chief rivers of the world has been estimated as below : 



Mississippi . 

 Upper Ganges 

 Hoang-Ho . 



Cubic Feet 

 7,468,694,400 

 6,368,077,440 

 17,520,000,000 



Cubic Feet 



Elione 600,000,800 



Danube 1,253,738,600 



Po 1,510,147,000 



The muddy condition of the water of certain rivers, caused 

 by this suspended matter, is so conspicuous a feature as to have 

 found recognition in the name applied. Hwang-Ho means simply 

 yellow river ; Missouri is the Indian name for Big Muddy ; while 

 the famous Eed River of the North is so called merely because 

 of the red mud it carries. Such silt-bearing streams, flowing 

 into lakes and tideless seas, begin depositing their loads so soon 

 as their currents are checked, building up thus the so-called 

 delta deposits for which the Mississippi, the Po, Ganges, and the 

 Nile are noted. 



The character of the material in the delta deposits is vari- 

 able only within certain limits, consisting always of siliceous 

 sand and mud intermingled with organic matter. 



Professor J. W. Judd found the materials of the Nile delta to 

 vary abruptly in texture from the surface downward, the varia- 

 tions following no recognizable law. The percentage amounts of 

 constituents classed as sand and mud, as obtained from (I) 

 borings at Kasr-el-Nil, Cairo, (II) Kafr-ez-Zayat, and (III) 

 Tantah, are given in the table on the next page. 



The material described as sand consists of rounded, angular, 

 and sub-angular grains. The well-rounded granules are mainly 

 of quartz and feldspar; the angular and sub-angular of quartz, 

 feldspars, hornblende, and augite, with smaller quantities of 

 mica, tourmaline, sphene, iolite, zircon, fluor-spar, and magnetite 

 all in a nearly unaltered condition. The feldspars are mainly 

 orthoclase and mierocline — rarely a soda-lime variety — and 

 in a state of surprising freshness. The quartz is in part the 

 quartz of granitic rocks and the larger grains well rounded, 

 best described as microscopic pebbles. ''It is evident that 

 these sand grains have been formed by the breaking up of granitic 

 and metamorphic rocks, or of older sandstones derived directly 

 from such rocks. The larger grains exhibit the perfect rounding 



