660 DYNAMICAL GEOLOGY. 



the Yang-tse, has no delta, but enters the ocean in one stream four 

 miles wide. The tide rises there about seven feet. 



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 reqnire 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. About one third of the area of the delta is a sea-marsh, 

 only two thirds lying above the level of the Gulf. Professor E. W. Hilgard has shown 

 that, about New Orleans, the modern alluvium has a depth of only thirty-one to fifty- 

 six feet, there existing below this the alluvial clay, etc., of the Port Hudson group 

 (p. 548). 



Deltas are formed from the conjoined action of the river and the 

 ocean, and are sometimes called fluvio-marine formations (p. 682). 

 The delta of the Mississippi (see preceding page, Fig. 1093) com- 

 mences below the mouth of Red River, where the Atchafalaya bayou 

 begins, which is the first of the many side-channels that open through 

 the great flats to the Gulf. The whole area occupied by it is about 

 12,300 square miles. Thus much has the river encroached on the out- 

 line of the Gulf; what amount in its depth, by its contributions of silt, 

 is beyond calculation. The material which reaches the Gulf is mostly 

 drifted westward toward Galveston. Other deltas, as those of the 

 Nile, Ganges, and other great rivers, differ little, in the instruction 

 they afford, from that of the Mississippi. 



The contributions of rivers have added largely to the seashore 

 plains of all of eastern North America, from Texas to Florida, and 

 from Florida to New Jersey, as illustrated under the subject of the 

 Ocean. 



5. Structure of Formations. 



(a.) Ordinary stratification. — Stratification is for the most part a 

 result of alternation in the conditions attending deposition. The varia- 

 tion from floods to low waters, and all changes in rate of flow, result 

 usually in making a succession of unlike layers. When a current car- 

 rying sediment is long sustained at a given rate, the deposit may have 

 throughout a uniform texture ; but only slight changes in rate will 

 cause changes in the depositions from coarser to finer, or the reverse. 

 Even the alternations of night and day may be registered if the waters 

 vary in amount through the melting of ice in the mountains. 



