


386 “DYNAMICAL GEOLOGY. —— [Boox III, 
sediment on the land is only temporary ; the inevitable destination 
of all detrital material is the floor of the sea. Most rivers which enter — 
the sea have their mouths crossed by a bar of gravel, sand, or mud. — 
The formation of this barrier results from the conflict between the © 
river and the ocean. Although the muddy fresh water floats on the 
heavier salt water, its current is lessened, and it can no longer push © 
along the mass of detritus at the bottom, which therefore accumulates — 
and tends to form a bar. It has been ascertained, moreover, that, 
though fresh water can retain for a long while fine mud in 
suspension, this sediment is rapidly thrown down when the fresh is — 
mixed with saline water. Hence, apart from the necessary loss of 
transporting power by the checking of the river current at the — 
mouth, the mere mingling of a river with the sea must of itself be a 
cause of the deposit of sediment. (See postea, p. 435.) Moreover, in — 
many cases the sea itself piles up great part of the sand and gravel 
of the bar. Heavy river-floods push the bar farther to sea, or even — 
temporarily destroy it; storms from the sea, on the other hand, drive 
it farther up the stream. 
Some of these facts in the régime of rivers have been well © 
studied at the mouths of the Mississippi. At the South-west Pass — 
the bar is equal in bulk to a solid mass one mile square and 490 feet 
thick, and advances at the rate of 338 feet each year. It is formed 
where the river water begins to ascend over the heavier salt-water of 
the gulf, and consists mainly of the sediment that is pushed along 
the bed of the river. A singular feature of the Mississippi bars is 
the formation upon them of “ mud-lumps.” These are masses of 
tough clay, varying in size from mere protuberances like tree trunks, — 
up to islands several acres in extent. They rise suddenly and attain 
heights of from 3 to 10, sometimes even 18 feet above the sea-level. 
Salt springs emitting inflammable gas rise upon them. After the 
lapse of a considerable time the springs cease to emit gas, and the 

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. :! i 
Fic, 125.—SmHme@_e and SAnpD-spit (e) AT THE MoutH or an Estuary (c), ENTERED 
BY A KiveR, AND OPENING UPON AN EXPOSED Rocky Coast-Line (B.). 
lumps are worn away by the currents of the river and the gulf. . 
The origin of these excrescences has been attributed to the generation 
of carburetted hydrogen by the decomposing vegetable matter in th 
sediment underlying the tenacious clay of the bars.? a 
* Humphreys and Abbot, “ Report on Mississippi River,” 1861, p. 452. 
