Page:Popular Science Monthly Volume 80.djvu/245

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THE MISSISSIPPI DELTA

accompanied by an occasional bubble of combustible gas. Remnants of such active mudlump cones have successively obstructed the Pass à l'Outre, its branchy the Northeast Pass, and the Southwest Pass. Rod soundings in active mudlump craters have reached a depth of 24 feet, but no solid bottom.

On measuring the proportion between the volume of gas and mud, I found the former to range from about one twenty-fifth to one thirtieth of the mud flow; the uniformity of which clearly indicates a steady pressure or vis-a-tergo. The mud flow, according to the universal testimony of river pilots, varies with the stages of the river, becoming much more lively at times of flood. Clearly, the gas is a wholly subordinate feature and not the cause of the mud flow; as has already been stated by Sir Charles Lyell. The latter attributed the ascent of the mud to the pressure of the sands and silts of the bar at the mouth of the Pass upon a mud stratum lying beneath them and under the bed of the river, the origin of which he, however, did not try to explain, but which is now to be considered.

Mud Layer Formed by Flocculation Beyond the Bar

My investigations of the peculiarities of colloidal clay, begun about 1869, led me to conjecture that the stratum of liquid mud was due to the precipitation of such clay from its diffusion in the turbid clay-water passing over the bar, by intermixture with the saline sea water; and that a layer of gelatinous, semi-fluid mud should, therefore, be found to seaward of the bar. That such is actually the case was proved by numerous reports received from pilots of sea-going vessels, who stated that at varying distances outside of the bar the sounding-lead begins to sink more slowly before it comes to a final stop on solid sea bottom, usually the Port Hudson clay. I could not obtain any definite estimate of the depth of the mudlayer, but the pilots said it might be from five to fifteen feet, according to the distance out from the bar.

I have, unfortunately, been unable to obtain an authentic sample of this mud from outside the bar. But of its existence there can be no doubt, and the huge scale upon which clay precipitation by flocculation occurs at mouths of all turbid streams emptying into the sea or saline basins, clearly shows that flocculation is certainly not the "limited and obscure" phenomenon that Chamberlin and Salisbury declare it to be ("Geology," Vol. I., p. 360). As the bar is built forward, the river sands and silts are spread on the mud stratum, so as to bury it under a broad cover. There would then tend to form on the surface of the mud stratum, by upward filtration, a thin but compact crust of tough clay constituting the bed of the river beneath the sands and silts which