864 



SCIENCE. 



[N. S. Vol. XXIV. No. 626. 



feet) shall we account for the continued ex- 

 istence of a fluid mud-layer for more than a 

 century, in the case of Morgan's lump — why 

 has not this mud been squeeezed dry into a 

 sheet of clay long ago ? 



In my view, the entire phenomenon rests 

 upon and is conditioned by the existence of 

 the ' blue delta clay ' and ' blue clay bottom ' 

 long commented on respectively by the engi- 

 neers in charge of the river work, and the 

 pilots off the Mississippi mouths. This blue 

 clay constitutes a shelf reaching out about 

 twenty-eight miles beyond the present mouths, 

 where there is a steep descent into deep water. 

 Wherever this clay is exposed along the gulf 

 shore, it contains cypress stumps and other 

 vestiges of swamp origin. It constitutes the 

 main body of the formation which in my 

 Mississippi report of 1860 is doubtfully desig- 

 nated as Coast Pliocene, but to which subse- 

 quently, finding it most characteristically de- 

 veloped at Port Hudson, I gave the latter 

 name. Its existence and nature imply that 

 swamp or marsh conditions prevailed to gulf- 

 ward for nearly thirty miles beyond the pres- 

 ent coast line, prior to the advent of the Mis- 

 sissippi River of to-day. Since that time the 

 land has been depressed and reelevated to the 

 extent of some 450 feet at least; but a prior 

 elevation, indicated by stream-gravel beds 

 now 450 feet below sea-level, must have thrown 

 the Mississippi Valley drainage northward 

 toward the Arctic, the divide between the two 

 drainages being very low. This former north- 

 ward direction of the drainage has been dis- 

 cussed somewhat widely before, by Tight and 

 others, but not, so far as I know, with special 

 reference to the gulf datum-plane and its 

 bordering formations. The lower Mississippi 

 River of to-day is evidently a very ' young ' 

 stream. 



However that may be, the blue-clay shelf is 

 there, and is practically water-tight and proof 

 against erosion so long as it remains sub- 

 merged and no gravel is carried by the cur- 

 rent. It is in this respect much like the mud- 

 lump clay itself. It is constantly found by 

 the sounding-lead outside of the river bar; 

 but before reaching it the lead sinks slowly 

 for some distance in a semifluid mud, which is 



undistinguishable from that flowing from the 

 mudlump vents. It is manifestly the result 

 of the precipitation of the finest clay and silt 

 when the river water mixes with that of the 

 sea. 



In its annual advance of 338 feet to sea- 

 ward, the sandy bar-material covers this fluid 

 clay much faster than the latter can escape 

 to seaward under the pressure. It is covered 

 by the heavy bar-sand, which from a boat in 

 the shallow water over the bar-crest can be 

 seen being carried rapidly over, shallowing the 

 water outside; while the deepest water inside 

 the har is found near the hose of its landward 

 slope. And it is just there, i. e., just inside 

 the bar, and not, as Lyell seems to imply, on 

 ' the adjoining bottom of the gulf,' that the 

 rise of mudlumps chiefly takes place; right 

 where the strongest current seems to indicate 

 the best channel for ships to pass. In other 

 words, the current excavates the river bed 

 immediately inside the bar, and, relieving the 

 superincumbent pressure, thus enables the 

 mud-bubbles to rise. 



It is not clear whether Lyell considers the 

 bar as such to exert the pressure causing the 

 rise; it is, at any rate, difficult to see how the 

 static pressure of a submerged bar could cause 

 the same material to rise from ten to fifteen 

 feet above tide-level.^ But there is no doubt 

 that the weight and wide base of the bar is 

 able to materially obstruct, if not prevent, the 

 squeezing of the semi-fluid mud to seaward, 

 despite a considerable vis-a-tergo from land- 

 ward. 



Given a semi-fluid layer of mud on an im- 

 pervious clay bottom, reaching as far land- 

 ward as Morgan's lump at least, the source of 

 pressure is not far to seek. The sediment 

 annually deposited on the marsh areas above, 

 with their heavy growth of rushes and other 

 aquatic vegetation, supplies ample weight; 

 and the statement of the pilots that the mud- 

 lump springs always become more active when 

 these marshes are overflowed, adds cogency to 

 this explanation. The vis-a-tergo is the ever- 

 in,creasing weight of the river-sediments 



^ The specific gravity of the outflowing mud 

 ranges from a minimimi of 1.25 to as much as 

 1.75. 



