862 



SCIENCE. 



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



the normal mode of progression of the delta of 

 the main Mississippi. 



No such phenomena are known to occur in any 

 other river of the world, hence no other river has 

 such birdfoot mouths. The Mississippi delta 

 should not, therefore, be longer presented as the 

 type of a normal delta, as is done by Eussell in 

 his * Rivers of North America.' 



The above paper was not written out for 

 publication, as I then thought it sufficient to 

 have called the attention of geologists to the 

 omission of so exceptional and unique a fea- 

 ture, already elaborately discussed by Sir 

 Charles Lyell in the tenth edition of his 

 ^Elements' (pp. 448-454), and belonging to 

 the principal river of North America. I find 

 the same misconception and omission, how- 

 ever, in Chamberlin and Salisbury's ' Hand- 

 book of Geology ' ; hence it seems desirable to 

 call attention more pointedly, as was suggested 

 to me by Eussell, shortly before his death. 



Upon the ' correction ' of the mouths of the 

 great river, rendered necessary by the peculiar 

 phenomenon of the mudlumps, the govern- 

 ment has in the past been obliged to spend 

 many millions. It is still compelled to keep 

 dredgers constantly at work, notwithstanding 

 the fact that the enormous volume of the 

 river has been turned into the single channel 

 of the ' south pass ' of the delta. Why does 

 not the scouring action of the current keep 

 this channel permanently open, once for all? 

 And why does not the river, in its twice- 

 annual overflows, break and wash permanent 

 lateral channels through the narrow barriers 

 or levees that jut out into the gulf in bird- 

 foot shape, unlike all other deltas in the 

 world ? 



A simple examination of the material of 

 which the banks of the ' passes ' and of the 

 uprising mudlumps in the channels are com- 

 posed, answers these questions categorically. 

 The material in both cases is a tough clay, 

 wholly unlike any of the visible sediment car- 

 ried by the stream; the latter forms only a 

 thin surface layer on the main clay mass 

 bordering and confining the currents of the 

 river, whose scouring action is powerless so 

 long as the clay remains permanently sub- 

 merged. 



Whence this clay, and why should it be 

 brought up forcibly from the channel of the 

 river, of all places the one where the scouring 

 should be most effectively done? 



Of this forcing-up there can be no question 

 whatever. Pilots and ship-captains have seen 

 the channel in which they passed to sea a 

 short time before, completely blocked by a mass 

 of tough clay on their return. A ship thus 

 run aground in several feet of water in the 

 evening has found its bow raised out of the 

 water in the morning, requiring several tugs 

 to pull it off backward. For years, tugs with 

 gigantic engines pulled the entering and the 

 sea-going ships through the tough mud, which 

 all the dredging and artificial scouring done 

 by the U, S. River Service could not control. 

 Frequently the mud kept rising as fast as the 

 dredger worked. 



Soundings around newly risen mudlumps, 

 and sometimes direct inspection, generally 

 show them to have the form of a rounded 

 bubble, from whose highest point there fre- 

 quently issue gas bubbles, and sometimes a 

 flow of liquid mud visible even below water. 

 Gas bubbles were also noted whenever the 

 dredgers disturbed a lump in the channel. 



When a mudlump rises above tide level, as 

 is frequently the case, there are usually 

 formed on its summit one or several vents, 

 like the craters of mud volcanoes, from which 

 there issues a steady flow of semi-fluid mud, 

 agitated from time to time by gas bubbles; 

 the gas is combustible and, as ascertained by 

 the writer, is such a mixture of marsh gas 

 and carbonic dioxid as is evolved from organic 

 debris in their first stages of decay. This gas 

 is undoubtedly derived from the large masses 

 of trees and other vegetable matter carried 

 and buried by the river in its deposits. But 

 its amount, as Lyell correctly estimated, is 

 wholly inadequate to account for the copious 

 and steady flow of fluid mud, which gradually 

 builds up flat cones of solidified material, 

 sometimes attaining the height of fifteen to 

 eighteen feet above tidewater, but more com- 

 monly six to ten feet. Usually other craters 

 are formed before the extreme height is 

 reached; or several mud-bubbles coalescing 

 may form a small island with several vents. 



