242 THE POPULAR SCIENCE MONTHLY 



are continually shifting with the current. Under this crust, however, 

 the semifluid mud-stratum would remain, and would also underlie the 

 river channel inside of the bar, on both sides of the channel, and 

 farther upstream to an unknown distance, in one connected layer; and 

 would be subjected everywhere to the pressure of the marshes and over- 

 flow materials. 



Mechanism of Mudlump Upheaval 



Taking the existence of this semi-fluid mud-stratum resting upon 

 the Port Hudson clay and buried by the delta sands and silts through- 

 out the lower delta, for granted, the mechanism of mudlump upheaval 

 at once suggests itself ; for the rapid advance of the heavy and extended 

 load of river sands and silts of the bar over the mud stratum would not 

 permit the escape of the slow-moving mud to seaward. 



A striking confirmation of the presumption that the pressure on 

 the mudlayer is exerted by the accumulation of sediment and vegeta- 

 tion in the marshes, and of the existence of the mud-layer itself even 

 under the older marsh formation, is the occurrence of a large mud- 

 lump-cone in full activity in 1867, in the marsh seven miles above the 

 mouth of the Southwest Pass, and between it and West Bay. From 

 the Pass, it appeared as a slightly irregular conical hill, which, judging 

 from the extent to which it projected above the highest reeds, was about 

 18 to 20 feet in height. A glittering mud stream on the south slope 

 could readily be distinguished by the field glass. The lump was inac- 

 cessible at the time of my visit, but had previously been fully described 

 from a personal visit by pilot Ben Morgan. 



Mudlumps commonly arise in a channel or pass immediately inside 

 of the steep upstream slope of the dar, in or alongside of the main cur- 

 rent, where the depth is greatest, and where the bottom therefore can 

 most readily yield. Soundings show that (doubtless owing to the im- 

 pact and consequent scooping action of the river current as it is forced 

 to ascend to the crest of the bar) there is nearly always a maximum 

 depth just at that point, in the course of the main channel. This would 

 seem to mean that as the weight of the superincumbent river sands 

 and silts is thus relieved, the pressure of the great area of marshes 

 lying upstream, of the delta deposits on either side, and to some ex- 

 tent, perhaps, the pressure of the bar itself, causes the upheaval of 

 the river bed and in many, though not in all, cases produces an ex- 

 trusion of the semi-fluid mud which is but slowly washed away by the 

 current. The vents are formed near the water surface at flrst, but are 

 then built up from the outflowing mud, which partly consolidates by 

 loss of water, until small mud volcanoes, rising usually from three to 

 four, but sometimes as much as twelve feet above the water surface, are 

 formed. 



