Page:Popular Science Monthly Volume 80.djvu/246

242 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 overflow materials.

Taking the existence of this semi-fluid mud-stratum resting upon the Port Hudson clay and buried by the delta sands and silts throughout 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 vegetation in the marshes, and of the existence of the mud-layer itself even under the older marsh formation, is the occurrence of a large mudlump-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 inaccessible 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 bar, in or alongside of the main current, where the depth is greatest, and where the bottom therefore can most readily yield. Soundings show that (doubtless owing to the impact 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 extent, perhaps, the pressure of the bar itself, causes the upheaval of the river bed and in many, though not in all, cases produces an extrusion of the semi-fluid mud which is but slowly washed away by the current. The vents are formed near the water surface at first, 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.