WATER. 647 



from Florida to New Jersey, is nearly a continuous range of fluvio- 

 marine formations. 



Only a single example — that of the Mississippi delta — need here be referred to. 



The preceding" map (fig. 942) presents its general features. It commences 

 below the mouth of Red River, where the Atchafalaya "bayou" begins, — the 

 first of the many side-channels that open through the great flats to the Gulf. 

 The whole area is about 12,300 square miles, and about one-third is a sea-marsh, 

 only two-thirds lying above the level of the Gulf. 



On page 643 the amount of detritus is mentioned which the river annually 

 furnishes towards 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 deposit, give for the difference between the cubical con- 

 tents 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 dis- 

 charge, 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 Sotithwest 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 

 interesting subjects of study. But it is not necessary to enter into details re- 

 specting them in this place, as they illustrate no new principles. 



As the forms and stratification of delta deposits depend partly upon wave- 

 action, this subject comes up again under the head of The Ocean. 



B. SUBTERRANEAN WATERS. 



It is an obvious fact that a considerable part of the water which 

 reaches the earth's surface descends into the soil and becomes in a 

 sense subterranean. But there are also subterranean streams, 

 which have their rise in hills and mountains, and are fed, like the 

 surface-rivers, by the rains and snows, and especially those that 

 fall about elevated regions. . These waters become under-ground 

 streams by following the clip of tilted strata. The layers of sand- 

 stones and limestones never fit together so closely but that waters 

 may find their way between them. The subterranean streams 

 usually flow over limestone or argillaceous strata, and not on 

 porous sandstones. 



