December 28, 1906.] 



SCIENCE. 



863 



The latter in the end are usually choked by 

 solidified mud; and so soon as this happens 

 the work of destruction begins. Sometimes 

 "the lump collapses bodily, segments of circular 

 fissures forming all over it, often leaving a 

 small lagoon in the middle; this occurs espe- 

 •cially where the material is not a very stiff 

 ■clay, as in the southwest pass. On the eastern 

 portions of the birdfoct area the destruction of 

 the lumps usually occurs by the waves washing 

 ■over the dried mud of the cones, and bringing 

 it down, to be partly washed away, partly de- 

 posited in the intervals between adjacent 

 lumps, connecting them and thus gradually 

 forming a solid, continuous clay dam, on 

 which the river current exerts no sensible 

 •eroding effects while it remains submerged. 



It is in this way that the narrow bands that 

 bound the outer passes of the Mississippi are 

 formed and maintained, and are made to 

 progress seaward. In other words, mudlump 

 formation is at present the normal mode of 

 progression of the visible delta into the gulf. 



How far back in time or distance this mode 

 of progression reaches is, of course, unknown 

 at present; but the peculiar, onion-like struc- 

 ture of mudliimps that have risen above the 

 •surface and become mud volcanoes, should 

 Tender this not a difficult problem to solve 

 where sections exist or shall be made within 

 the delta. The peculiar material, so different 

 from any now deposited by the river above its 

 mouth, and which in fact can only have been 

 deposited in slack-water, is enough to dis- 

 tinguish the mudlump formation. It is this 

 same clay that has for ages withstood the 

 impact of the main river-current at the ' head 

 of the passes,' whence the several outlets 

 diverge; the same material forms the narrow 

 banks of the ' neck,' at the head of which, 

 where it diverges from the main delta-mass, 

 forts Jackson and St. Philip are located. 

 Were it otherwise, the narrow barrier sepa- 

 rating the neck from Garden-island Bay could 

 not have survived a dozen years of floods; 

 whereas even the channels purposely cut 

 through it by duck-hunters to avoid the long 

 detour through the passes into the bays, have 

 hardly been enlarged in fifty years. 



If any more evidence were needed, it is 



supplied by the existence of an active mud- 

 volcano in the marsh, seven miles above the 

 mouth of the southwest pass, where the writer 

 saw it spouting mudspatters and emitting 

 mudstreams in 1869. This mudlump (then 

 known as Morgan's lump) projected at least 

 eight feet above the tall rushes (Scirpus lacus- 

 tris), and rose, therefore, at least sixteen feet 

 above the level of the marsh. 



Sir Charles Lyell ('Elements,' etc., tenth 

 edition, p. 452) inclines to carry the mud- 

 lump-genesis of the delta as far up as New 

 Orleans, from information given him by Col. 

 Sidell, of the River Service, U, S. Topograph- 

 ical Engineers. It should not be difficult to 

 verify this in excavations made at New Or- 

 leans; mere borings can not, of course, de- 

 termine the question. 



As to the origin of the mudlumps, Lyell 

 Qoc. cit.) considers them to be formed on the 

 principle of the * creeps ' so familiar to engi- 

 neers and miners; he justly ascribes only a 

 secondary part to the gases brought up with 

 the mud, which according to my measure- 

 ments amount to only one twentieth to one 

 thirtieth of the volvune of the material ejected. 

 Lyell says : " The initiatory power may prob- 

 ably be derived from the downward pressure 

 of the gravel, sand and sediment accumulated 

 during the flood season off the various mouths 

 or passes, upon a yielding bottom of fine mud 

 and sand; materials which, as being very fine 

 and impalpable, had long before been carried 

 out farthest from the land." The great mass 

 of river sediment " may well be conceived to 

 exert a downward pressure capable of dis- 

 placing, squeezing, and forcing up laterally, 

 some parts of the adjoining bottom of the 

 gulf, so as to give rise to new shoals and 

 islands." 



There can be no question of the general 

 correctness of Lyell's explanation of this phe- 

 nomenon, which certainly constitutes the most 

 gigantic example of creep known, and as such 

 should concern the geologist quite as much as 

 the engineer. For why should the Mississippi, 

 of all rivers in the world, alone exhibit this 

 remarkable feature and mode of progression? 

 and how, in view of the known average annual 

 progression of the delta into the gulf (338 



