652 



DYNAMICAL GEOLOGY. 



On p. 649, the fimouift of detritus is mentioned which the river annually furnishes 

 toward the extension of the delta. 



According to Humphreys & Abbot, the outer crest of the bar of the Southwest Pass 

 {the principal one) of the Mississippi advances into the Gulf 338 feet, over a width of 

 11,500 feet, annually; and the erosive power is only about one tenth of its depositing 

 power. The depth of the Gulf, where the bar is now formed, being 100 feet, the profile 

 and other dimensions of the river, in connection with the above-mentioned rate of de- 

 posit, give for the difference between the cubical contents of yearly deposit and erosion 

 255,000,000 cubic feet, or a mass one mile square and nine feet thick: this, therefore, 

 is the volume of earthy matter pushed into the Gulf each year at the Southwest Pass. 

 The quantities of earthy matter pushed along by the several passes being in proportion 

 to their volumes of discharge, the whole amount thus carried yearly to the Gulf is 

 750,000,000 cubic feet, or a mass one mile square and twenty-seven feet thick. As the 

 cubical contents of the whole mass of the bar of the Southwest Pass are equal to a solid 

 one mile square and 490 feet thick, it would require fifty-five years to form the bar as 

 it now exists, or, in other words, to establish the equilibrium between the advancing 

 rates of erosion and deposit. 



The deltas of the Nile, Ganges, Amazon and other large streams are equally interest- 



