THE MISSISSIPPI DELTA 241 



accompanied hj an occasional bubble of combustible gas. Eemnants of 

 such active mudlump cones have successively obstructed the Pass a 

 rOutre, its branchy the Northeast Pass, and the Southwest Pass. Eod 

 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 uni- 

 versal testimony of river pilots, varies with the stages of the river, be- 

 coming 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 heneath 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 Layee Formed by Flocculation Beyond the Bak 



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, there- 

 fore, 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, unfortunatel}^, 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 



VOL. LXXX. — 17. 



