>/ 
 
 v 
 
ON THE 
 
 GEOLOGY OF THE DELTA, 
 
 AND THE 
 
 MUDLUMPS OF THE PASSES OF THE MISSISSIPPI. 
 
 BY EUG. W. HILGAKD. 
 
 [Read before the American Association at its last meeting in Troy.] 
 
[FROM THE AMERICAN JOURNAL OP SCIENCE AND ARTS, VOL I, 1871.] 
 
 In previous papers* read before this Association, I have com- 
 municated the results obtained during two successive geological 
 expeditions to Louisiana, so far as the more ancient formations 
 are concerned. It is the object of the present communication 
 to present and discuss the phenomena of that portion of the 
 *. Jj^f pi tetyj explored by me, lying within what is usually con- 
 "'si&feficL .the alluvial area, proper or Delta, of the Mississippi 
 .pweii .Mosrt of these observations were made in 1867, during 
 " * the, jji^t-.of. "these excursions referred to, under the auspices of 
 the Smithsonian Institution ; their publication having been 
 delayed in consequence of want of time, on my part to carry 
 out the large amount of chemical and microscopic work in- 
 volved in the discussion, which is even yet far from being as 
 full as would be desirable. I hope, however, to be enabled, 
 hereafter, to continue the investigation of the subject, both in 
 the field and in the laboratory. f 
 
 I. The Upper Delta. Plain. 
 
 I recall to mind the fact that, as we descend the Mississippi, 
 the older strata successively sink from view. A few miles 
 below Vicksburg we lose sight of the older Tertiary. Thence 
 down to the latitude of Tunica Bend, La., we find the rocks 
 of the Grand Gulf (Tertiary) age, possessing but a very faint 
 southward dip. Next, the oldest representative of the quater- 
 nary epoch, viz., the stratified Drift or Orange Sand, disappears 
 beneath the water's edge near Port Hudson ; while the swamp, 
 lagoon and fluviatile beds, which have given notoriety to the 
 latter locality, are seen above high water level not much farther 
 south than the city of Baton Rouge. 
 
 The gradual descent and successive disappearance of these 
 strata is not, however, altogether a phenomenon of dip, in the 
 
 * This Journal, II, vol. xlvii, Jan., 1 869 ; Ibid, xlviii, Nov., 1 869. 
 
 f For material assistance in the investigations embraced in this paper, I am 
 especially indebted to the officers of the Delta Survey in charge of the Coast Sur- 
 vey schooner Varina Messrs F. P. Webber and Henry L. Marinden, for reports 
 of observations, and specimens furnished; to Capt. Day, in command of the same 
 vessel, then moored at the Head of the Passes, for a boat's crew and personal 
 assistance in the examination of the mudlumps ; to Mr. Moulton, of the Cromwell 
 line of steamers; to Captain Ed. Yorke, of the Towboat Association, and to 
 Capt. Andrews, then of the dredgeboat at the Southwest pass, for free transporta- 
 tion on their respective vessels ; and to Capt. Tilford and other gentlemen of the 
 N. 0. Pilot Association, for generous hospitality as well as much valuable informa- 
 tion. For similar favors I am indebted to Dr. Copes, President of the N. 0. Acad. 
 of Sciences, as well as other members of that body. Other acknowledgments 
 will be found in their proper place. 
 
and the Mudlumps of the Mississippi. 339 
 
 usual sense, but in a great measure the result of consecutive 
 deposition ; while the surface slope of the Port Hudson deposits 
 is manifestly in a great degree due to denudation, and in part, 
 no doubt, to deposition on a sloping bottom. At Port Hudson 
 as well as elsewhere, where extensive profiles can be seen, the 
 deposits of that era exhibit the basin shape ; both on the large 
 and, sometimes, on the small scale. In this as well as in their 
 lithological and paleontological features, they greatly resemble 
 deposits now forming over large areas ; and where the two are 
 in juxtaposition, it is often difficult, sometimes impossible, to 
 draw the line between them, since qualitatively their process of 
 formation has been manifestly the same. Both above and 
 below Port Hudson, and down as far as Fort St. Philip, the 
 apparently alluvial river banks frequently exhibit at low water 
 edge, solid blue clays, with cypress stumps and twigs im- 
 bedded therein, scarcely distinguishable from some materials 
 occurring at Port Hudson, Cote Blanche, and other localities 
 of the Port Hudson age ; and as neither can be expected to 
 contain any but living organisms, it is, thus far, from general 
 considerations alone, that we can hope to deduce their real age. 
 It might seem, at first sight, that the distinction is practically 
 of little moment; but when it is considered that the Port 
 Hudson deposits are separated in time from those of the pres- 
 ent era, by a large portion of the " Champlain " period of 
 depression, plus the entire " Terrace " period of elevation, it 
 becomes obvious that the distinction is one of no little theoret- 
 ical, and some practical, interest. For while the Port Hudson 
 strata yield to the augur almost invariably a considerable rise 
 of artesian water, no such result can usually be looked for in 
 either river or delta deposits. 
 
 I think that a retrospective view of the geological history 
 of the lower Mississippi Valley and Gulf Coast, as developed 
 by my observations in the States of Mississippi and Louisiana, 
 will serve to show the probability that by far the greater portion 
 of what now constitutes the alluvial plain of the lower Missis- 
 sippi, is covered by the river deposits to a comparatively insig- 
 nificant depth only ; excepting where the ever shifting river 
 channel itself has caused an unusual depth by excavation and 
 subsequent filling up. 
 
 I have shown that toward the close of the Drift period, the 
 place of the present Mississippi was occupied by what, but for 
 its stupendous proportions, might be termed a torrent of fresh 
 water, having, even as far south as the present coast line, a 
 velocity sufficient to transport pebbles of five to six ounces 
 weight, from localities not nearer than Tennessee and northern 
 Arkansas; together with the smaller ones derived, doubtless, 
 
 970962 
 
340 E. W. Hilgard Geology of the Delta, 
 
 from the same sources as the drift boulders of Missouri and 
 Illinois.* 
 
 In the bored wells of Calcasieu, these pebbles have been 
 found as much as 450 feet below tide level ; the inevitable 
 inference being (provided the sea level remained constant), 
 that since the time of their transportation, the coast has suf- 
 fered a depression to at least that extent in matter of fact, 
 probably, fully twice that amount f The thickness of the 
 drift stratum is about one hundred feet, the materials growing 
 finer toward the top ; indicating, therefore, a diminished 
 velocity of the depositing current. 
 
 Overlying this sand and pebble drift, we find, as at Port 
 Hudson, alternating strata of more or less lignitiferous clay 
 and sand, 850 feet thick; a 130-foot clay bed lying on top.- 
 In its uppermost portion, this bed recalls to mind at once the 
 variously colored clays of the Cote Blanche profile, with 
 their calcareous concretions; but here, according to the con- 
 current testimony of the inhabitants and the microscope, marine 
 shells take the place of the fresh-water fauna observed at the 
 former locality. The sands which, on the whole, predominate 
 in the lower portion, are readily distinguished under the micro- 
 scope from those of the drift, by the predominant sharpness of 
 the grains, and the occurrence of particles of lignitized wood ; 
 but I have not thus far succeeded in finding in them any other 
 organisms. They resemble strikingly the materials obtained at 
 corresponding depths in the bored wells in the city of New 
 Orleans. 
 
 Few deep wells exist near the coast, in the interval between 
 that city and the Calcasieu bores. In the only one of which I 
 possess definite data, viz., at Salt Point on Bayou Sale, a bed 
 of marine shells was found after passing through the first clay 
 bed, at 60 feet. 
 
 * Miss. Rep., 1860, p. 26 andff.; this Journal, II, May, 1866; and Nov., 1866; 
 Ibid Jan., 1869; Ibid, Nov., 1869. 
 
 The magnitude of this phenomenon would give it a continental significance 
 even if it were confined to what I have termed the " Orange Sand delta," below 
 Cairo. But as I have elsewhere stated, similar pebhle streams, with slack water 
 deposits of ferruginous sands intervening, exist in Texas ; and I have the satis- 
 faction of adding to the array of facts heretofore presented in connection with 
 this subject, the testimony of Prof. Safford, of Tennessee; who, upon discussion, 
 finds that the phenomena presented by the " Bluff gravel," "Ore region gravel," 
 and '-Eastern gravel " of his report are most satisfactorily accounted for, and 
 brought under a common point of view, upon the basis set forth in my publica- 
 tions, above referred to. 
 
 In connection with Tuomey's and my own observations in Alabama, and farther 
 east, those of Prof. Safford acquire additional significance, and the southern strati- 
 fied drift an additional cLiim upon the serious attention of American geologists. 
 Compare Prof. Newberry's interesting paper on "The Surface Geology of the 
 basin of the Great Lakes, and the Valley of the Mississippi," where it is stated 
 that "no deposits corresponding to the drift of the northern and western states, 
 exist south of the Ohio river," (Ann. Lye. Nat. Hist, N. York, ix, 213, 1869). 
 
 f This Journal, II, Nov , 1869, p. 335. 
 
and the Mudlumps of the Mississippi. 341 
 
 The same is true of the formation skirting the coast of Mis- 
 sissippi Sound. Deep wells there sometimes, though not al- 
 ways, strike beds of marine shells, and water possessing a con- 
 siderable rise, after penetrating the uppermost clay bed ; which 
 there usually also contains cypress stumps, and forms the " blue 
 cla^y bottom " of the Gulf coast. Sometimes, though rarely, de- 
 posits of marine shells, of living species, appear in the beds of 
 streams. But so far as I know, their occurrence is limited to 
 within a moderate distance from the general coast line ; so that 
 the great body of the formation underlying the upland parishes 
 of East Louisiana, the Attakapas and Calcasieu prairies, as 
 well as the coast-belt of prairies in Texas, consists of marsh, 
 lagoon, and fluviatile deposits, with, probably, many an inlet 
 or estuary of a more or less brackish or marine character. 
 
 Obvious as is the conformation of this littoral belt to the 
 outline of the Gulf coast, the Mississippi valley influences it 
 only in so far, as that its strata have here, probably, their 
 highest absolute elevation,* and farthest extension northward. 
 The latter circumstance is the natural consequence of the exist- 
 ence of the depression which, at least since the opening of the 
 Cretaceous period, has determined the outline of the formations 
 southward of Cairo an embayment which, with every succeed- 
 ing period of deposition, became less concave, until at the 
 close of the Grand Gulf epoch, the concavity had all but dis- 
 appeared. During the period of slow depression which charac- 
 terized the Port Hudson era, the present general coast line must 
 have been established ; and when upon the reversal of the 
 movement of subsidence, the waters of the continent began to 
 be discharged through what is now the lower Mississippi valley, 
 the erosion seems to have been checked everywhere, save per- 
 haps in the main channel, by the tough cypress swamp clay 
 which now forms the immediate substratum of the lower littoral 
 belt, and extends far into the waters of the Gulf. 
 
 It is incredible that the deposition which occurred along the 
 whole Gulf coast from Indianola to Mobile, should not have 
 taken place also in the main axis of the depression which, as 
 the trend of the formations shows, had in a great measure been 
 filled up. Yet, inasmuch as this was the deepest portion of the 
 area, it is to be expected that here, if anywhere, marine deposits 
 should extend far inland. We shall not therefore be surprised 
 to find that, as Pourtales has proven from the soundings made 
 under the direction of Gen. Humphreys, the Mississippi river 
 flows on marine beds, at New Orleans and Bonnet Carre. It 
 would rather be remarkable if such beds should not appear 
 even much higher up the river, since even in the comparatively 
 
 * As regards the main body. At Weeks' Island and Petite Anse, and proba- 
 bly at Cote Blanche and Orange Island, their elevation exceeds that at Port Hud- 
 son bluff. 
 
342 & W. Hilgard Geology of the Delta, 
 
 insignificant valley of Pearl river, they have been met with in 
 the latitude of Baton Rouge,* about 30 miles from the coast. 
 
 It is, doubtless, owing to the formation of these swamp 
 deposits, and their subsequent resistance to denudation during 
 and since the Terrace epoch of elevation, that the main body of 
 the truly alluvial delta is thrown so far beyond the general coast 
 line,f out into the Gulf. It is the shallow "blue clay bottom," 
 so well known to navigators on the Gulf coast, which forces 
 the great river to advance its mouths so rapidly toward deep 
 water, by the accumulation of its own deposits ; and the bor- 
 ings made at New Orleans have shown how slight is the thick- 
 ness, even at such an advanced point, of the river deposits 
 proper, overlying the older formation. 
 
 I owe to the active interest taken in this subject by Gen.. 
 A. A. Humphreys, U. S. A., an opportunity of examining, 
 not only the specimens collected during the boring of the 
 artesian well at New Orleans by a committee of the New 
 Orleans Academy of Sciences (so far as they were preserved 
 from destruction during the war) ; but also those obtained in 
 the soundings made by the delta survey under his charge, upon 
 which a very able and minute report of a microscopic examina- 
 tion had previously been made by Mr. L. R Pourtales. Upon 
 the strength of the data furnished by the latter, as well as by the 
 profile constructed by the committee of the Academy (repro- 
 duced in the " Report on the Physics and Hydraulics of the 
 Mississippi river "), Gen. Humphreys concluded that at Bonnet 
 Carre and New Orleans, the river flows on an ancient sea bot- 
 tom, which he conjectured to be of Tertiary age. Sir Charles 
 Lyell having questioned the correctness of this view, Gen. 
 Humphreys obtained from the N. 0. Academy as complete a 
 suite of specimens of the borings as could be collected, and 
 referred them to me for examination. The first results of this 
 investigation are given, in substance, in the first volume of 
 Ly ell's Principles of Geology, 10th edition, p. 459 ; they were 
 based substantially upon the determination of the visible shells 
 (mollusks) contained in several of the specimens, embracing, 
 fortunately, most of the important horizons mentioned in the 
 profile. I have since gone over the whole ground, in the micro- 
 scopic examination of all the available specimens, with a view 
 to determining their (marine or fresh water) character, and the 
 admissibility of the supposition that they might belong to the 
 delta formation proper. 
 
 My detailed report of this examination will, I presume, be 
 published before Iong4 Unfortunately, most of the specimens 
 
 * Miss. Rep., 1860, p. 156. 
 
 f Drawn, say from the mouth of Pearl river to Belle Isle, the most advanced 
 outpost of the Port Hudson deposits on the Louisiana coast ; which line will pass 
 near the city of New Orleans. 
 
 \ In Rep. of the U. S. Engineer Dept., for 1870. 
 
and the Mudlumps of the Mississippi. 343 
 
 representing the important clay strata (of 34, 32-J, 39, and 63-J, 
 feet respectively), were missing ; but as regards the rest, 51 in 
 number, I found almost all derived from a lower level than 
 SI feet, either characterized by marine organisms (shells, corals 
 or foraminifera), or of such a character as, by their obvious 
 connection with the others, to put them in the same category, 
 although devoid of fossils. I cannot omit to mention in this 
 connection, the extraordinary scarcity of marine organisms in 
 some of the specimens brought up in sounding off, on and 
 inside the bars of the Missiasippi Passes ; the most patient 
 search, even after concentration by washing, having failed to 
 bring to light anything but minute fragments of wood, root 
 and other vegetable fibers, and remnants of Naviculas. These 
 specimens had, it is true, been obtained during a period of high 
 water ; and others, collected at corresponding points but at a 
 different season, showed abundance of foraminifera and even 
 some visible shells. But in view of these facts, it is quite in- 
 telligible how in an estuarian formation, forming at the outlet 
 of the continental waters, many portions may have remained 
 destitute of any vestige of marine life, though perhaps deposited 
 in strongly brackish water ; while any slackening in the rate of 
 depression would promptly cause a predominance of the fresh 
 over the salt water, a stagnation, and consequent deposition of 
 clays ; which would be nearly or quite free from vestiges of 
 marine life. Such is the case in some of the few clay specimens 
 from this bore, which I have had the opportunity of examin- 
 ing; but others I have found to contain not only foraminifera, 
 but abundance of shells. In the clay occurring at the greatest 
 depth reached 630 feet Mr. Pourtales reports an abundance 
 of foraminifera. 
 
 The specimens from the several beds contained altogether 
 about 50 species of mollusks, of which 40 were in such a con- 
 dition as to be determinable with certainty. Of these, 36 were 
 species now living in the Gulf, and 4 were new. 
 
 The latter (belonging to the genera Gardium, Abra, Semele 
 and Tapes) I submitted to Mr. Conrad for determination and 
 description. He remarks that while they (one of them espe- 
 cially) seem to be rather of miocene type, and not known to be 
 now living in the Gulf of Mexico : yet our knowledge of the 
 fauna of the latter is so imperfect thus far, that it cannot be as- 
 serted that the species are not now inhabitants of the Gulf 
 waters. 
 
 As regards the distribution of the species, there is no mate- 
 rial difference from the highest to the lowest level, the leading 
 and predominant species being everywhere about the same, and 
 coinciding in a marked manner with the fauna collected by 
 myself on the beach of Ship Island in the Mississippi Sound ; 
 
344 K W. Hilgard Geology of the Delta, 
 
 though quite different in the prevalence of species, from that 
 now cast ashore on the islands of the delta. One of the new 
 species, moreover, occurs abundantly in one of the very first 
 shell-beds ; and three of them at the depth of 235 feet, as well 
 as, in part, still lower down. 
 
 In view of all the facts bearing on the case, the most probable 
 conclusion is that the marine formation penetrated in the New 
 Orleans well is altogether independent of the present delta for- 
 mation ; that, on the contrary, it is the equivalent in time of 
 the Port Hudson deposits, which everywhere near the coast 
 assume a marine facies ; and would necessarily possess that 
 character in an increased degree, where the deepest depression 
 existed. 
 
 The thickness of the alluvium proper in the alluvial plain 
 will, of course, vary in accordance with the degree of denuda- 
 tion that the older formation may have experienced during the 
 era of upheaval ; and it is futile to attempt an estimate of the 
 amount of alluvium deposited by the great river since the 
 beginning of the modern era, until numerous observations 
 shall have placed us in possession of data allowing us to form 
 an approximate estimate of its depth in the several portions of 
 the alluvial plain. While there exist in it, doubtless, a num- 
 ber of ancient river channels, we already have proof also of 
 the existence of ridges of more solid and ancient ground, far 
 out in the delta plain, which seem to have caused the eastward 
 deflection (parallel to the Teche and the main Mississippi), of 
 Bayou Lafourche as well as of the minor channels. I have 
 heretofore* alluded to the apparent general cause of this de- 
 flection, viz., the barrier of drift materials accumulated, 
 perhaps, upon a Cretaceous nucleus, which is presented by the 
 chain of Five Islands Belle Isle, Cote Blanche, Weeks' Island, 
 Petite Anse and Orange Island extending from Atchafalaya 
 to Yermilion Bay. 
 
 The very variable depth of the alluvium is well exemplified 
 by the borings made for water and gas, in the city of New 
 Orleans, by Mr. J. B. Knight, of that city. Its lower limit 
 seems to be almost everywhere marked by a stratum of liquid 
 mud, beneath which appears the first shell bed. It is from this 
 mud stratum, which is struck at depths varying from 31 (in 
 the well of 1856) to 56 feet, that combustible gas is frequently 
 found to issue in considerable abundance, and with a pressure 
 (as reported by Mr. Knight) of from 1-J to 3 pounds per square 
 inch. The discovery (which was made by Mr. Knight in bor- 
 ing an experimental well for water, on his premises) at first 
 created considerable excitement, as it was thought 'the natural 
 gas might successfully compete with that of the gas company, 
 
 * This Journal, II, Jan., 1869, p. 88 ; ibid, Nov., 1869, p. 343. 
 
and the Mudlumps of the Mississippi. 345 
 
 which it was said to equal in quality. That this was but an 
 indifferent compliment to the company's product, may be 
 judged from the composition of the natural gas, which was 
 analyzed by Prof. J. W. Mallet, then of the University of 
 Louisiana, with the following result : 
 
 Gas from "Knights 1 well" 170 Gravier street. New Orleans. 
 
 Marsh gas, 91 '81 
 
 Carbonic acid, _ 2 '97 
 
 Nitrogen, 5 -32 
 
 Hydrocarbons condensible by bromine, trace 
 
 100-00 
 
 The gas issued at the rate of one and a half cubic feet per 
 hour, with a pressure of 1 '6 inches of mercury. It was reached 
 at a depth of 40 feet, and was accompanied by a considerable 
 flow of faintly saline water. * 
 
 Mr. Knight sunk numerous wells in different portions of the 
 city, and states that gas was struck nearly everywhere at depths 
 varying from 37 to 56 feet ; its amount, in one and the same 
 region, being sensibly proportional to the diameter of the bore. 
 In a few cases, an extraordinary amount of gas, under strong 
 pressure, was struck. In an article published in the New 
 Orleans Times, of March 19, 1870, it is stated that " at the old 
 Washington Artillery building on Girod street, a pipe was 
 driven for water, and the gas flowed through in such a 
 volume, that when ignited it fed a flame 15 feet in height, 
 which was with' difficulty extinguished ; and when it was at 
 length choked out, it carried up several cart loads of sand in a 
 single night. Yesterday a similar phenomenon presented itself 
 on the edge of the sidewalk in Camp street, opposite Lafayette 
 square. A pipe about an inch and a half in diameter had 
 been driven into the ground for water, when at the depth of 
 60 feet, a rush of gas, accompanied by water and sand, was 
 forced through to a distance (height) of twelve or fifteen feet 
 above the top of the pipe. This continued for two hours, in 
 spite of all efforts to suppress it ; and the result was a deposit 
 of sand mixed with fine shells and pebbles, amounting to at 
 least three cart loads. Finally the workmen succeeded in 
 closing the pipe, and forcing it through the gas-bearing 
 stratum." 
 
 The supply of gas was in the end, however, thought inade- 
 quate for practical purposes ; while the original object of obtain- 
 ing drinkable water was attained to a limited extent only. 
 Mr. Knight says in a letter on the subject, that he has "found 
 great irregularity in the thickness and character of the strata, 
 
 * See analysis beyond. 
 
346 E. W. Hilgard Geology of the Delta, etc. 
 
 and it is impossible to tell the character of the water or the 
 depth at which it will be found, before trying. At two places 
 distant about 1,500 feet, I obtained, in one, at the depth of 
 48 feet, a free supply of clear water strongly impregnated with 
 iron ; in the other, no such water could be found at the depth 
 of 78 feet." 
 
 Attention having been called to the subject, a " find " of gas 
 was next reported from the parish of Lafourche, said to rise 
 with a pressure of 10 pounds to the inch. I have been unable to 
 learn whether or not this occurrence of gas is identical with that 
 described to me by Col. Thibodeaux, of Thibodeauxville, as 
 keeping up a continual agitation of the waters of the "Bayou 
 bouillant," on the lower Lafourche. So far, no practical appli- 
 cation of this source of gas has come to my knowledge. 
 
 As regards, then, the upper delta plain, there can be little 
 doubt that, like the Calcasieu and Attakapas prairies, it is 
 underlaid by the detrital deposits of the stratified Drift, at a 
 depth which may fairly, d priori, be supposed commensurate, in 
 a measure, with the importance of the neighboring channels ; 
 viz., the Sabine on one hand, and the Mississippi on the other. 
 About midway between, the Cretaceous ridge marked by the 
 line of outliers from Lake Bisteneau to Chicotville or Petite 
 Anse, has caused these same deposits to appear at the surface.* 
 The overlying swamp, lagoon and estuarian deposits of the 
 Port Hudson age, will vary both in thickness and in the 
 (marine or fresh-water) character of their materials., in accord- 
 ance with the conformation (relative to the ocean) of the surface 
 upon which they were deposited. And the alluvial deposits 
 proper will, in like manner, vary in thickness in accordance 
 with the degree of denudation previously experienced by that 
 older formation, but appear to be little greater on the alluvial 
 plain near New Orleans, than it is sometimes found to be in the 
 Yazoo and Tensas bottoms, f 
 
 It is important to note that, under this point of view, the 
 ultimate success of an artesian bore at New Orleans becomes a 
 matter of certainty a question of depth alone. All water 
 obtained in the Port Hudson strata possesses considerable rise, 
 but is usually too strongly mineral to be desirable for every- 
 day use. The waters obtained in the Orange Sand, on the con- 
 trary, are always remarkably pure, and when struck beneath the 
 Port Hudson deposits cannot fail to possess a proportionate 
 rise, as in Dr. Kirkiman's bore, on the West Fork of Calcasieu. 
 The waters of the Port Hudson strata would, of course, require 
 to be tubed out. 
 
 [To be continued.] 
 
 * This Journal, II, Nov., 1869, pp. 332, 342 and ff. 
 f Humphrey's and Abbot's report, pp. 98-100, et al. 
 
and the Mudlumps of the Mississippi. 347 
 
 II. The Lower Delta and the Mudlumps. 
 
 A glance at the map shows that in descending the Mississippi 
 from New Orleans, we find a narrow strip of land only f to 3 
 miles wide, dividing the river from the waters of the Gulf; from 
 the head of Oyster Bay opposite Poirite a la Hache (about half 
 way between the city and the head of the Passes), down to 
 the mouths. Such, at least, is the case on the left bank ; on the 
 right, the "neck" begins a few miles below Fort Jackson. 
 Down to the forts, the aspect of the " Coast" is generally pretty 
 much the same, where its original character has not been lost by 
 cultivation or encroachment of the river. Nearest the river, 
 and highest above water level, are the sandy " willow bat- 
 tures," where the willow, mingled with and occasionally re- 
 placed by the cottonwood. forms the predominant growth. Be- 
 yond lies a belt of woodland, timbered chiefly with live-oak, 
 magnolia, and cottonwood, often deeply veiled with long- moss ; 
 this belt embraces the richest and most durable soils of the 
 " Lower Coast," and is mostly occupied by magnificent planta- 
 tions of sugar cane and orange orchards. Beyond these, loom 
 in the distance the sombre-hued, moss-curtained denizens of 
 the cypress swamp, their tops forming a level platform sharply 
 defined against the horizon. Between the swamp and the wa- 
 ter's edge, seaward, there usually intervenes a zone of reeds, 
 with here and there a stunted cypress, bay, or candleberry bush, 
 where the salt water has but slight access. 
 
 While such is the general order of succession of these belts 
 of vegetation where they coexist, either or both of the two 
 middle ones may locally be absent Such is alwaj-s the case 
 where the u neck" is very narrow, as happens below the forts. 
 Thence to the mouths of the passes, the willow batture and the 
 reed marsh alone, with few exceptions, form the barrier be- 
 tween the river and the sea ; it is traversed by numerous small 
 bayous, some of which are in great part the work of the duck- 
 hunters that supply the New Orleans market, and whose pur- 
 suit leads them to penetrate the marsh for the purpose of reach- 
 ing the favorite resorts of their game. These bayous increase 
 in frequency as we descend, and in approaching the mouths of 
 the passes, the intervals between them become smaller, until 
 they gradually become sheets of water dividing islands ; and 
 finally, just inside the bar, we have the latter resolved into nu- 
 merous individual "mudlumps," dotting the surface of the sea, 
 on both sides of the main channel. 
 
348 E. W. Hilgardr Geology of the Delta,, 
 
 Sir Charles Lyell remarks (Principles of Geology, 10th ed., 
 p. 448), that the "phenomenon of the mudlumps is without par- 
 allel, so far as known, in the delta of any other river. The 
 same remark might, I think, appply to two other peculiarities, 
 viz : the protrusion of the long neck of land into the Gulf ; 
 and the fact that, after failing to send out any branch of impor- 
 tance for a hundred miles the great river suddenly divides at 
 one point into three widely divergent branches, the middle one 
 of which (the South Pass), forming the direct continuation of 
 the channel, is the smallest, and has long ceased to be navigable. 
 Evidently, a strong extraneous obstacle alone could turn aside 
 the powerful current, and permanently resist its erosive and un- 
 dermining action. And now, the channel which carries the 
 main current (the Southwest Pass), faithful to the old tradition, 
 is rapidly pushing out into the Gulf its narrow bands of reedy 
 marsh, without a branch of any consequence in ten miles from 
 the head of the Passes to the light-house. 
 
 A glance at the coast lines, as well as at the intricate ramifica- 
 tions characterizing the deltas of the Ehine, the Po, the Danube, 
 the Ganges, or the Hoang-Ho ; or the broad inlets forming the 
 mouths of the rivers of South America, will show the unique- 
 ness of the Mississippi mouths ; the Nile and the Lena alone 
 exhibiting a general form at all analogous, yet very distinct in 
 detail. For the islands off the Lena mouths are not " mud- 
 lumps ;" and the tongue of land separating Lake Menzaleh from 
 the Damietta branch of the Nile, is a mere sand-bar, exhibiting 
 no analogy save that of form, with the remarkable u necks" of 
 the Mississippi Passes. 
 
 It would be fair to infer, a priori, that some connection ex- 
 ists between the exceptional phenomenon of the mudlumps, 
 and the exceptional form of the delta ; and that such is really 
 the case, can hardly be doubted upon a candid investigation of 
 the facts. So far from being an unusual phenomenon, the mud- 
 lump-formation appears to constitute the normal mode of progression 
 of the 1 Mississippi mouths ; not only at the present time, but for 
 many ages past ; perhaps ever since the broad flood of the Terrace 
 epoch subsided into the present Mississippi. 
 
 The characteristic features of the mudlumps have successively 
 been described and discussed by Sidell,* Forshey,f Chase. 
 Beauregard and Latimer,$ Thornassy and LyellJj Yet as the 
 phenomena are nowhere described in their entirety, I will here, 
 as briefly as possible, recapitulate the important points. 
 
 * Report to Capt. Talcott, 1839, in Humphreys and Abbott's Report, App. A. 
 f MS. Reporr. 1830. 
 
 j Keport of the Board of Engineers for the Examination of the Mississippi; Con- 
 gress. Doc. 1852-53. 
 
 Geologic Pratiqu-de IM Louisiane, 1860, Chap. VI. 
 
 I Principles of Geology. 10th edition, 1868; vol. 1. p. 449, 
 
and the Mudlumps of the Mississippi. 
 
 849 
 
350 E. W. Hilgard Geology of the Delta, 
 
 The mudlumps originate in upheavals of the bottom, in the 
 region lying between the extreme point of the mainland and 
 the crest of the bar, at the main outlets of the passes. They 
 all lie within a distance of from one to three miles from the 
 axis of the main current, and nowhere extend into the bays in- 
 tervening between the several active mouths. The pilots are 
 under the impression that they form more frequently in the 
 main channel than elsewhere ; but allowance must be made for 
 the natural infrequency of their observations outside of the latter ; 
 so that only the more obvious and remarkable changes of the 
 bottom, when occurring outside of their regular range, would 
 be likely to come under their notice. A priori, it would seem 
 probable that inasmuch as the resistance to the upheaving force, 
 other things being equal, must be less in deep than in shallow 
 water, the lumps would rise more frequently and more rapidly 
 in the channel than elsewhere. On the other hand, the denu- 
 ding force of the river current must greatly diminish the chan- 
 ces of any such upheaved mass appearing above the surface, or 
 even becoming manifest by a diminution of depth, when pro- 
 gressing slowly. Weighing these considerations against each 
 other, I think a balance remains in favor of the pilot's opinion ; 
 the more as concurrent testimony goes to show that whenever 
 the channel is changed, either by the accumulation of deposit or 
 the interference of a mudlump upheaval, the old channel is 
 promptly occupied by mudlumps throughout the abandoned 
 portion. 
 
 As regards the rapidity of the upheaval, it appears to vary 
 from an almost secular rate, to that of several feet in 24 hours. 
 It has been rumored that lumps had been seen to rise visibly, 
 and burst open like a bubble, but I have been unable to trace 
 the statement to any reliable source, and it is discredited by the 
 pilots. The most rapid rise of which I have obtained reliable 
 information was witnessed by Capt. Andrews, of New Orleans. 
 He states that a steamer having run aground about nightfall in- 
 side the bar, her bow being on a mudlump in about 2-J feet of 
 water, he towed her off during the night, and in the morning- 
 found the imprint of her bow 18 to 24 inches above water, in the 
 soft mud. Allowing for the possible maximum influence of 
 tides in favor of the difference observed, the minimum rate of 
 upheaval, according to these data, still amounts to one inch per 
 hour. Nor is this the only case in which a rapid shallowing of 
 the water has been observed as a consequence of the grounding 
 of large vessels. This has usually been ascribed to the acci? 
 mulation of deposit in the slack water so formed, and in many 
 instances this may have been the true cause. But this explana- 
 tion can hardly apply to the case detailed above, and it is rea- 
 sonable to suppose that the shock of a grounding vessel may 
 
and the Mudlumps of the Mississippi 851 
 
 in many cases prove " the hair that breaks the camel's back," 
 when acting upon a portion of bottom just about to yield to 
 the upheaving force, and therefore in a state of unstable equi- 
 librium. Future observations, however, will readily settle the 
 question. 
 
 Of authentic observations illustrating ordinary rates of rising, 
 I quote the following remarks of Mr. H. L. Marindin, U. S. 
 Coast Survey, who in 1867, while engaged in the survey of the 
 mouths, at my request paid considerable attention to the phe- 
 nomena of the mudlumps, and made an interesting report to me 
 on the subject, dated June 20th, and accompanied by speci- 
 mens. 
 
 " Since the first examination of the mudlumps on Southwest 
 Pass Bar, there have appeared numerous lumps in the vicinity 
 of the main channel, whose appearance cannot be attributed to 
 other causes than the upheaval of the bottom by some unknown 
 agency ; since tidal observations made during the month of April, 
 May and part of June, show no material decrease in the height of 
 water in the river, other than the daily ebb of the tide. In one 
 instance, also, on this bar, it has been ascertained from surveys 
 made at different times, that where in the month of April a 
 channel with fifteen feet of water existed, there is now a lump, 
 doubtless still forming, with only six feet of water on it at the 
 highest stage." 
 
 Making allowance for a possible accumulation of deposit, the 
 rate of rise cannot, in this instance, be estimated at less than two 
 feet per month ; which, so long as the lump remains under wa- 
 ter, seems to be no unusual amount. When, however, the sur- 
 face of the water is once reached, the rising becomes slower or 
 ceases altogether, owing probably to the greater weight acquired 
 by the material upon its emergence. At least, I have failed to 
 find much above tide level, any material which did not bear 
 distinct evidence of its having been formed, not by river allu- 
 vion, but by the action of the regular mudsprings ; which, if not 
 now active upon all the lumps, have demonstrably been instru- 
 mental in forming the great majority of the masses now above 
 water level. 
 
 I do not know that any one has ever witnessed the first out- 
 burst of a mudspring on a newly risen lump ; but we find them 
 in all stages of progress, from the islet bearing its first tuft of 
 rushes, to the active cones glistening in the sunshine, and from 
 time to time, when an unusually large gas bubble rises, spatter- 
 ing the liquid mud (which usually flows in a quiet stream) all 
 over the slopes ; then the half-extinct cone, in whose crater a 
 little pool of salt water is at long intervals agitated by a gas 
 bubble ; then the extinct and collapsed cone, surrounded by a 
 circular moat and Somma-like ring-wall ; next, the old lump of 
 
352 E. W. Hilyard Geology of the Delta, 
 
 jagged outlines, whose disintegrated materials are rapidly yield- 
 ing to the combined attack of rain, sunshine and waves, till 
 something looking like a large stump of a tree is all that re- 
 mains of an island of several acres ; and finally the shoal, marked 
 by dangerous rollers, whose surface, on a calm da}', still exhibits 
 the concentric markings corresponding to the several cones 
 which originally built up the island. 
 
 I regret being unable at the present time, to present accurate 
 sketches of these several stages of " the mudlumps' progress." I 
 hope to do so hereafter, but for the present must confine myself to 
 the approximate outline representations given in the plate, and 
 a brief description of the several stages as observed by myself in 
 1857. 
 
 Nascent Lumps. As regards the first, it is probable that a 
 good many lumps never pass beyond that stage of develop- 
 ment, tor the reason that, so soon as the resistance is materially 
 increased by the emergence of a portion above the surface of 
 the water, the upheaving force seeks a vent elsewhere. 
 
 Either the mud or gas-spring breaks out beneath the water, 
 and becomes perceptible only by the more or less regular and 
 localized evolution of bubbles on the outskirts of the lumps ; 
 a very common phenomenon in the neighborhood, not only of 
 new and active lumps, but also about extinct ones, which are 
 in course of demolition by the waves. On the extensive sandy 
 shoal off Stake Island, on the Southwest Pass, such subaqueous 
 gas-springs may be observed in great numbers. Or it may 
 happen, that another portion of the bottom, now offering less 
 resistance than the mudlump, will, in its turn, give way before 
 the upheaving force, till the same degree of emersion is ob- 
 tained, or a vent is opened. 
 
 I have been unable to ascertain how high any mudlumps can 
 rise bodily above the water without the appearance of a vent 
 As much as three feet has been observed with certainty ; but 
 unless the fact that it is a new upheaval be historically known, 
 it must be extremely difficult to ascertain it, unless, by actual 
 access to the interior, it can be shown that its surface strata are 
 old river deposits, which can readily be distinguished from 
 those formed by mudlump vents. Yet these might, during the 
 elevation of the lump, have been removed bv the current At 
 all events. I have failed to find on the surface of any lump 
 much above tide- water, anything like true river alluvium ; the 
 visible material being either such as is now formed by active 
 vents, or, (as on the lower slopes), that which obviously results 
 from the disintegration of the former, being altogether devoid 
 of structure. 
 
 It is said that lumps sometimes sink from view again after 
 bare emergence. I know of no authentic example, but it 
 
and the Mudlumps of the Mississippi. 353 
 
 seems likely enough that upon the formation of a large vent else- 
 where, such a thing might happen ; especially if, as I think 
 probable, mudsprings and mud-cones form beneath, as well as 
 above the water. 
 
 Active Cones. In the second stage, that characterized by the 
 formation of the active eruptive cones, mudlumps offer an as- 
 pect so strikingly like that of the mud volcanoes of Tuscany, 
 as to stagger the observer's geographical consciousness. The 
 most striking example of the kind existed, at the time of my 
 visit, off Pass a 1 'Outre, on the south (right) side of the channel. 
 The island* is about an acre in extent, and besides the active 
 vents (of which there are seven), we perceive on the southern 
 portion the remnants of long extinct craters, in various degrees 
 of advancement toward old age and degradation, and partially 
 covered with vegetation. 
 
 The cones on the north or channel side, at a distance, present 
 a glistening surface, and those possessing a central vent only 
 are very regularly and smoothly conical. Of these there are 
 four, the diameters of the bases ranging from eighteen to fifty 
 feet, and their elevation from one to two and a half feet above 
 the general level of the island ; making the angle of the slope 
 from 5 to 8 only, instead of 25 to 35, as given in the 
 published sketches. I have seen no undisturbed cone whose 
 slope exceeded about 12. The slope is, of course, ' essentially 
 a function of the thickness and character of the mud ; which in 
 the present instance flows from the circular basin at the summit, 
 4 to 8 ins. in diameter, in a regular, creamy stream, interrupted 
 occasionally only by a gas bubble ; which, if the mud be thick, 
 spatters it about to some extent. The mud-stream varies from 
 3 to 4 gallons per minute in the largest (eastern) cone, to a pint 
 or two in the less active ones ; sometimes, in running down the 
 sides, it spreads over as much as -J- of the circumference, but 
 usually forms a stream 4 to 8 inches wide, serpenting down the 
 slope- between banks formed by its own solidification, as does a 
 lava stream. As these banks grow in height by drying, on the 
 edge of the crater, they gradually compel the mud to rise 
 higher before it can flow off; whereupon, after a while the 
 column overflows at another point of the circumference, where 
 the same play is then repeated. In the meantime, the previous 
 mudstream has an opportunity of consolidating, drying more 
 or less, and undergoing a variety of chemical changes depen- 
 dent upon the character of the water and the duration of the 
 exposure to the air, which manifest themselves in changes of 
 color and consistency ; by these the individual streams are 
 distinctly defined from one another. Each one forms, of course, 
 
 * It was named Marindin's Lump by the crew of the U. S. Coast Survey Schr. 
 Varina, and I shall allude to it under that designation. 
 
354 E. W. Hilgard Geology of the Delta, 
 
 a more or less irregular portion of a conic surface, the 
 section being more or less lenticular. 
 
 There results, as may be supposed, a very peculiar structure or 
 stratification, unlike anything usually seen outside of volcanic 
 districts, unless, perhaps in the somewhat analogous case of the 
 lee side of dunes. It may roughly be compared to the upper 
 half of an onion. It is needless to say that, once seen, it cannot 
 readily be mistaken for anything else ; and its absence must be 
 held as proof conclusive of an absence of genetic analogy.* 
 The thickness of the layers rarely exceeds 1^ inches ; their 
 colors vary from dark mouse-color and bluish gray, through 
 dun and buff, to rust-color and red ; the materials, from hard, 
 plastic clay, always containing a great deal of fine silex and 
 more or less coarse sand, to sandy clay and, rarely, clayey 
 sand ; the selvedges are frequently marked by iron rust and 
 mica scales. Where, as is frequently the case, neighboring 
 cones are confluent at their bases, the structure is of course 
 complicated in a manner readily imagined. 
 
 As regards the gas evolved (which is in all cases inflammable), 
 the small proportion its bulk bears to that of the mud simul- 
 taneously ejected (about ^ to ^ at Marindin's Lump), at once 
 does away with the impression mostly entertained, that the gas 
 bubbles bring up the mud with them. The latter comes up 
 with a steady flow, evidently the result of static pressure, and 
 is only from time to time agitated by a gas bubble, larger or 
 smaller according as the mud is more or less consistent. We 
 sometimes find, about extinct cones especially, lively gas 
 springs in which the proportion of gas is considerably greater, 
 than happens in any active cone that has come under my 
 observation ; but in that case, it is almost always accompanied 
 only by water, as though its source Were above the stratum 
 which furnishes the mud. For, a glance at the river deposits 
 forming around the lumps,, at once shows that they are totally 
 different from the fine, clayey material of which the cones are 
 formed ; nor is it credible that the mere passage of a current 
 of water and gas through such deposits, or in fact, any consolidated 
 materials, should produce such a perfect, creamy mixture as 
 that ejected from these craters. 
 
 Extinct Cones. (rood examples of cones whose activity has 
 nearly or quite ceased, may be seen off the Northeast Pass. 
 Here the material is somewhat sandier and firmer than the 
 mud ejected at Passe a 1'Outre. While at the latter place it 
 was difficult to reach the craters without being hopelessly 
 
 * Vide Thomassy's remarks on the Five Islands, etc., Geol. prat, de la Louisiane, 
 chap. viii. With even less show of reason a similar origin has repeatedly been 
 claimed for the low circular mounds which dot a large part of western Louisiana. 
 They consist exclusively of unstratified sandy materials, and are doubtless the 
 result of animal activity probably of the large ant. 
 
and the Mudlumps of the Mississippi. 355 
 
 bogged, they can readily be reached almost dry shod at the 
 Northeast Pass. Here, also, the top layers were peeling off 
 in large concave " flakes," from the effect of sun-cracks and 
 drying ; such a surface again overflowed by mud would add 
 another singular feature to the structure of the cones, which 
 may also be noticed in many sections of ancient ones. 
 
 The vent of a large cone in this region (see Plate) formed a 
 basin about 8 inches diameter, containing a puddle or salt water 
 covered with an iridescent ferruginous pellicle, disturbed at long 
 intervals by small gas bubbles. Evidently, the resistance in the 
 large cone, elevated about 10 feet above the water level, had 
 become too great for the ejection of mud ; in consequence 
 whereof, a lively little cone was glistening and babbling at a 
 level several feet lower, near the foot of the old cone. But the 
 testimony of the pilots goes distinctly to show, that the active cones 
 become more lively, and dormant cones resume their activity, at high 
 stages of water in the river. At the time of mv visit, the water 
 was at an unusually low stage. 
 
 Collapsed Cones. The cones, especially the larger ones, appear 
 to be unable to survive for any great length of time the cessa- 
 tion of activity. The change that occurs seems to be, in all 
 cases, a sinking of the central portion, often to such an extent 
 that its place becomes occupied by a pool or lagoon, surrounded 
 by a circular rim whose strata incline away from the center at 
 angles invariably much steeper than is found in any recent 
 cones, from 20 to as much as 45. Outside of this^first rim 
 there mostly appears a series of concentric crevasses, sometimes 
 several feet in width and in depth ; and the annular segments 
 thus brought to a level by a subsidence, also exhibit the 
 singular feature of a steeper inclination of the lines of deposi- 
 tion, than is found in any cones now forming. The seeming 
 anomaly of this fact caused me to observe the phenomena 
 closely ; but I have been unable to find an exception to the 
 rule ; and I have been led to doubt whether i n some cases, 
 instead of true eruption cones, these collapsed areas and their 
 surroundings may not be the remnants of the original up- 
 heaved "bubble.' 1 The objection to this interpretation is the 
 character of the material, which, excepting in a few cases on 
 the Southwest Pass, is altogether unlike the present river 
 deposits, both in structure and composition. 
 
 A most perfect exemplification of a large central lagoon 
 (100 feet by 75) surrounded by an elevated rim and several 
 successive, concentric "moats," also in part filled with water, 
 occurs on one of the numerous mudlumps S.W. of Stake 
 Island, on the Southwest Pass (see Plate). The central portion 
 does not always, however, sink out of sight ; sometimes a conical 
 mound is still observable, as the center of the more or less circular, 
 
 c2 
 
356 K. W. Hilgard- Geology of the Delta, 
 
 concentric crevasses, which a close inspection shows to exist on 
 almost all lumps not too far advanced towards decrepitude. 
 Many islands exhibit several such centers and systems of 
 crevasses, indicating, probably, the previous existence of cones, 
 sometimes several hundred feet in diameter, greatly exceeding 
 the largest now in a state of activity, both in diameter and 
 original height. 
 
 Degradation and Disappearance of Lumps. The direct action 
 of the waves, unassisted by atmospheric agencies, produces but 
 little effect upon the yielding, but tough and coherent material 
 of the cones. The heaviest breakers and rollers spend their 
 force in vain against the clay shoals which render most of the 
 lumps so difficult of access. The unwary will often be tempt- 
 ed by the deceptive, rojk-like aspect of this material, to jump 
 into the shallow water and wade ashore ; but a plunge knee- 
 deep into the apparent solid will frequently reward his temerity. 
 Yet so long as this mass, which shows plainly the peculiar, 
 concentrically banded mudlump structure, remains constantly 
 covered with water, the waves rolling over it produce but little 
 impression. 
 
 Not so with the portion that projects above water, which is 
 alternately exposed to rain, sunshine, and the wash of waves. 
 Especially where the material is clayey, these agencies com- 
 bined soon produce a change in which the structural as well as 
 the paleontological characters of the original material are 
 totally obliterated. A rain falling upon a fresh surface of the 
 latter, causes it to swell ; then, upon exposure to sunshine, it 
 will contract into prismatic cleavage-forms. A slight rain, or 
 the spray itself, will then cause the extreme surface to crumble 
 into, and partially fill up, the cracks ; when the swelling 
 consequent upon a thorough wetting, by either rain or waves, 
 will force them to open still more, while streamlets of fluid 
 rnud follow each retiring wave; which, perhaps, has thrown 
 up, and left high and dry in the cracks, a dozen species of 
 shells, entirely foreign to the mucllump mud itself. It is thus 
 that the structureless, tough soil of the general surface, and of 
 the beach of the mudlumps, is formed ; and to it alone apply 
 the descriptions given of the mass of the mudlumps, by Sidell, 
 Thomassy and Lyell, as a "homogeneous, tenacious mud." 
 
 A frequent repetition of this process involves, of course, not 
 only a rapid direct degradation of the lump, but it causes it to 
 be cleft into fragments by rents gradually progressing from 
 above downward, which rapidly increase the surface exposed 
 to attack ; and eventually, some large wave, in retiring, carries 
 down with it a huge prismatic slice, leaving behind an almost 
 vertical cliff. This is carried away in its turn, and thus, 
 attacked from all sides, an island of considerable size, after 
 
and the Mudlumps of the Mississippi. 357 
 
 passing through a variety of middle stages in which it strik- 
 ingly resembles masses of trap or basalt, magnified into distant 
 mountains by the peculiar optical delusion prevailing in the 
 region,* is finally reduced to what, at a distance, appears to be 
 the stump of a tree. At last, some storm sweeps away this last 
 monument of the disappearing lump, and white-capped rollers 
 alone mark, thereafter, the higher points of the mud-shoal. 
 
 I have best observed these phenomena of disintegration in 
 every stage of progress, among the mudlumps off the Northeast 
 Pass, where the fanciful, cliff-like forms, figured by Thomassy, 
 Sidell, and Lyell, may be seen to perfection. 
 
 It is said that there are no mudlumps off South Pass or 
 Grand Bayou ; a statement which may require to be taken with 
 a grain of allowance, but agrees with the general impression 
 that the mouths discharging the largest amount of water, also 
 exhibit mudlump activity on the most extensive scale. 
 
 The Southwest Pass is the main outlet at the present time ; 
 the area inside the bar is thickly studded with mudlumps, 
 chiefly west of the channel ; and as before stated, lumps have 
 risen there repeatedly under the eyes of the pilots and survey 
 parties. Yet there is not there, at the present time, a single 
 active cone, so far as I am aware ; although salt water and gas 
 springs are of frequent occurrence, both on and around the 
 islands. There is a marked difference between the river 
 deposits as well as the mudlump materials of Passe a 1'Outre 
 and Southwest Pass, the latter being decidedly more sandy, 
 and sand bars taking the place of the mud flats off the former. 
 Whether this circumstance (the natural result of the greater 
 swiftness of the current in Southwest Pass), is connected with 
 the absence of active cones, it may be too early to discuss. 
 Some very lively springs on a large mudlump off Stake Island 
 on that Pass, in which the gas emitted is about equal in bulk 
 to the water, rise in small basins excavated at the foot of a 
 large cone which must have been 15 to 18 ft. high ; but the 
 material they bring up is so very sandy that the water runs off 
 perfectly clear.f 
 
 Mudlumps in the Marshes. I have before remarked, that at 
 the present time, the upheaval of mudlumps on the passes, and 
 subsequent silting up of the shallows between them (by river 
 deposit, as well as by the degradation of the lumps themselves,) 
 seems to be the normal mode of progression of the delta. The 
 more advanced portions of the narrow bands of shore now 
 
 * Caused probably by a faint bluish haze, through which an island a quarter of 
 a mile distant and 15 feet high, appears to be a wooded mountain with rocky 
 escarpments, and at its foot a wide spreading city which suddenly resolves itself 
 into a row of grave white pelicans perched on the beach and taking wing at the 
 approach of a boat. 
 
 f See analysis of the water of these springs, below ; marked u S. W. Pass, I." 
 
358 E. W. Hilgard Geology of the Delta, 
 
 forming along each one of the passes, are historically known to 
 consist of mudlump-chains ; and in the absence of any plausi- 
 ble presumption to the contrary, as well as of any parallel 
 example in other rivers, it is reasonable to surmise, that not 
 only the shores of the present passes, but also the neck, at least 
 from Pointe a la Hache down, owes it formation and peculiar 
 features to the same agencies. 
 
 We have seen how rapidly and completely the joint action of 
 the waves and atmospheric agencies accomplish the degradation 
 of elevated lumps to the common level of the tide ; and where 
 the nature of the materials is such as to yield readily to these 
 destructive influences, it would be unreasonable to look for 
 vestiges of ancient lumps above that level. Such is the case, 
 as before mentioned, on Northeast Pass and its branches.* 
 But the sandier nature of the mudlump mass on Southwest 
 Pass enables it to resist much longer, so that some of the larger 
 and more elevated islands there seem destined to retain, more 
 or less permanently, their present form. For the same reason, 
 perhaps, the Southwest Pass furnishes the one prominent ex- 
 ample of the existence of an active and characteristic mudlump, 
 in the level marsh on the right of the channel, about five miles 
 below the Head of the Passes, and seven above the mouth 
 (light-house) ; distant 1-J- miles from the river bank, and about 
 one mile from the beach of West Bay. Double-headed Bayou, 
 or one of its channels, passes within a few hundred yards 
 of this lump ; which is so difficult of access that it has been 
 very rarely visited, though plainly visible from the hurricane 
 deck of passing steamers, from which I have examined it with 
 the telescope. 
 
 According to Thomassy,f the pilot Ben. Morgan, who has 
 visited it, describes it as being " a regular truncated cone, 20 to 
 25 feet high and 300 in circumference, spouting at intervals 
 from its summit masses of clayey and sandy mud, which over- 
 flows all around." 
 
 From the Pass, it now appears as a slightly irregular, conical 
 hill, which, judging from the extent to which it projects above 
 the highest reeds, is about 18 to 20 feet high at most. On its 
 eastern side there is a second cone about half as high, with a very 
 regular slope not exceeding 30, while that of the large one is 
 at least 45. A whitish sheen which extends from the summit 
 of the larger cone toward the smaller one, I interpret as a 
 white saltv efflorescence ; but on the south slope of the smaller 
 
 * CoL Sidell mentions a mudlump cone, 18 feet high, on the north side of the 
 Northeast Pass, in the marsh. It has probably succumbed since his visit, as in 
 passing the spot I was unable to perceive any elevation, nor was it known to the 
 pilots. 
 
 f Geol. prat, de la Louisiane. p. 56. T shall designate this cone as ' J Morgan's 
 Lump." 
 
and the Mudlumps of the Mississippi. 359 
 
 cone, the glittering of a flowing mudstream was unmistakable. 
 Inasmuch as in Morgan's account of his visit the smaller cone 
 is not mentioned, it is presumable that it has been formed since, 
 by a lateral eruption ; the old cone having, perhaps, reached 
 the extreme limit of height to which mudlump force can raise 
 its materials. 
 
 The steep slopes of both the old and new cone are suggestive 
 as to the influence of sandiness on that feature, and the explana- 
 tion of the steep inclination of strata, observed chiefly on 
 Southwest Pass. 
 
 It is my impression that another cone exists in the marsh 
 about two miles south of these. It is almost screened from 
 view by the reeds, but the telescope shows it to be distinctly 
 conical. It does not seem to have attracted notice heretofore. 
 
 But if mudlump cones are scarce in the marshes, the same 
 is not true with reference to the salt and gas springs, which are 
 reported to be quite abundant by the hunters the only men 
 whose occupation leads them to "thread the pathless waste" of 
 reedy marsh, otherwise seldom visited, save by surveying par- 
 ties, and alligators. These springs are found on or around all 
 mudlumps, of whatever age ; even on the shoals left behind by 
 disintegrated lumps, where they issue under water, sometimes 
 altering perceptibly the character of the water in the immediate 
 neighborhood. The Southwest lighthouse was originally built 
 on a mudlump separated from the mainland by a bayou ; this 
 is now filled up, but salt springs still issue at several points in 
 the marsh near the foot of the tower. It is obvious that the 
 gradual accumulation of deposit is not likely to check lively 
 springs, possessing sufficient head to rise, hydrostatically, above 
 the level of the alluvium ; though in many cases they may lose 
 themselves in the sandy strata. 
 
 I have not had an opportunity of ascertaining whether or not 
 salt springs are known to exist in the marshes near the Forts, 
 and above. I fully expect to find, however, that they do exist, 
 though, for obvious reasons, they will become less and less 
 abundant as we ascend the river. At New Orleans, as already 
 stated, gas and salt water are reached, and brought to the sur- 
 face with considerable vehemence, by bores varying from 31 to 
 56 feet ;* and I have no difficulty in believing in the correct- 
 ness of the impression made upon Col. Sidell, that the founda- 
 tions of the New Orleans customhouse were located upon a 
 mudlump. f That such obstinate resistance as that of the " Head 
 of the Passes " to denudation can hardly be attributed to a mass 
 of river deposit, I have already intimated. A large mudlump 
 mass has, probably, first caused the deflection. 
 
 [To be continued.] 
 * See above, p. 245. f Lyell's Principles of Geol., 10th ed., p. 552 
 
E. W. Hilgard Geology of the Delta, etc. 360 
 
 Origin of the Mudlumps. The causes which give rise to the 
 formation of mudlumps have been to some extent discussed by 
 Sidell, Thomassy, and Lyell (loc. cit.). The former is inclined to 
 ascribe the upheaval chiefly to the pressure of gas formed in the 
 decay of driftwood and the like, buried in the river deposits. 
 Thomassy resorts to the hypothesis of the existence of subterra- 
 neous channels communicating with the river, or with equally 
 hypothetical reservoirs of water, far above ; while I/^ell ascribes 
 the bulging of the bottom to the pressure of newly formed de- 
 posits upon a substratum of yielding mud, accompanied, and 
 aided incidentally only, by the evolution of marsh gas in the 
 decay of organic matter. I myself, having become aware of 
 the existence of a strong artesian water pressure in the littoral 
 formations of the Gulf, was inclined to ascribe the origin of the 
 upheaving force to that source ; and my visit to the mouths had 
 for its object mainly, the comparison of the facts with each of 
 the three admissible hypotheses, that of Thomassy being too 
 fanciful to be seriously entertained. 
 
 As already stated, I at once found that the evolution of gas 
 in the active vents was too insignificant to be considered as the 
 cause of the rising of the liquid mud, which so greatly exceeded 
 it in bulk, that the ascensional force of the bubbles, especially 
 in so wide a vent-tube, would be utterly inadequate to balance 
 the downward tendency of so heavy a liquid. It might still be 
 alleged, in favor of the gas-hypothesis, that its pressure might 
 be exerted statically upon the surface of the mass of liquid 
 mud covered by impervious strata ; but it is obvious that in 
 such a case, the gas itself, necessarily accumulating at the 
 highest, and therefore weakest, points, of the superincumbent 
 mass, would be much more likely to break through by itself, 
 promptly exhausting its force and quantity at any one point 
 No such rushing outbreaks of gas have ever been recorded, 
 save in the case of blowing up of a lump with gunpowder; and, 
 as Lyell remarks, this view renders inexplicable the occurrence 
 of lumps exclusively about the mouths of the passes. 
 
 The latter objection applies equally to the hypothesis of the 
 artesian origin of mudlump force, unless upon the (unproved) 
 supposition that the excavation of the river channel might 
 have rendered the outbreak of the artesian water easier there 
 than elsewhere. But instead of excavating, the Mississippi has 
 for a long time past always thrown shallows in advance of its 
 
 AM. JOUR. Sci. THIRD SERIES, VOL. I, No. 6. JUNE, 1871. 
 28 
 
361 E. W. Hilgard Geology of the Delta, 
 
 mouths ; and unless it were conclusively proven that the mat- 
 ters ejected by the mud-springs were such as could not originate 
 in the present delta formation, the artesian hypothesis must lose 
 all show of probability. An accurate investigation of the mat- 
 ters in question, solid, liquid, and gaseous, was therefore 
 indicated. A few specimens for this purpose were collected for 
 me by Mr. Marindin, in 1867 ; but the perusal of his report 
 accompanying them convinced me that a personal examination 
 iTi loco could alone insure a perfect certainty as to their signifi- 
 cance, and accordingly, in the autumn of the same year, I 
 re-collected specimens from the same, as well as from other 
 localities. Yery unfortunately, the arrangements for gas analy- 
 sis at my command were so imperfect that, while waiting for 
 their improvement, the gas specimens were so vitiated by diffu- 
 sion through corks and wax as to render them useless ; and I 
 have been unable to replace them as yet, but hope to do so in 
 the near future. 
 
 Mudlump Gases. The examination of the water and mud 
 seemed, however, most likely to conduce to a solution oi the 
 problem, at any rate ; for after all, the only information which 
 could be furnished by gas analysis would be to indicate, by the 
 greater or less amount of carbonic acid present, whether the gas 
 originated from matter comparatively fresh and in its first stage 
 of decomposition, or had its source in materials far advanced 
 toward the stage of lignite or coals. The only perfectly reliable 
 determination made was that of the carbonic acid contained in 
 the gas collected from the most easterly active cone on Marin- 
 din'sLump, Passeal'Outre, the rest of the determinations being 
 somewhat vitiated, though doubtless very nearly correct* The 
 result was as follows : 
 
 Gas from East Crater, Marindin's Lump, Passe d V Outre. 
 
 Carbonic acid, 9'41 
 
 Marsh gan, 86'20 
 
 Nitrogen, 4'39 
 
 ] 00-00 
 
 Oxygen was not present. 
 
 The percentage of carbonic acid in this gas. is very unusually 
 large ; its composition is nearest to that of the gas from com- 
 mon swamps, where vegetable matter is in its first stages of de- 
 cay. The proportion between marsh gas and nitrogen is nearly 
 the same as in the gas from the gas wells at New Orleans (see 
 p. 245) ; but there is three times as much carbonic acid present 
 
 * After the explosion in the eudiometer, some nitrate of mercury was obserred on 
 its walls, in consequence of inadequate dilution of the gas. But the marsh gas 
 was estimated from the carbonic acid absorbed after the explosion, the nitrogen by 
 difference. 
 
and the Mudlumps of the Mississippi. 362 
 
 in the mud-lump gas, in accordance with the presumable more 
 advanced stage of decay existing in the former locality. 
 
 Mudlump Spring Waters. In taking specimens, common 
 quart bottles were filled by immersion in the craters them- 
 selves, and irfcmediately sealed. The liquid mud thus obtained 
 would, after a while, separate into a lower stratum of pretty 
 solid mud, and an upper one of clear water, in varying propor- 
 tions. For analysis, the latter was carefully decanted, and the 
 turbid part rapidly filtered through a Bunsen's pressure filter, 
 and measured. 400 ccm. were then boiled to precipitate car- 
 bonates and silica, the filtrate re-diluted to the original bulk, 
 and from 50 to 100 ccm. used in the determination, in separate 
 portions, of chlorine, of lime and magnesia, and of sulphuric 
 acid and alkalies, respectively ; while a fourth portion served 
 for an approximate determination of the solid residue, for the 
 sake of roughly controlling the final results.* 
 
 I give below, in tabular form, the results of these analyses ; 
 presented in three different forms, for the sake of ready com- 
 parison with the composition of sea-water, from, which they 
 seem to be derived by a series of reactions easily understood 
 from the nature and condition of the materials with which they 
 are associated. 
 
 I. Water from the basin of a spring on a mudlump off Stake 
 Island, Southwest Pass. Evolves gas and water in about equal 
 proportions, no mud, but only fine sandy matter ; and water- 
 flows off clear over the rim of the basin, wnich is two feet above 
 tide level, and at the foot of a large extinct cone with a lagoon, 
 surrounded by a high rim, in the center. 
 
 Water about J of the bulk in bottle, the rest fine sand. 
 Taste, very salty ; color, slightly yellowish ; turns brownish 
 turbid very quickly on exposure to air. Coll. Dec. 3, 1867. 
 
 II. Water from a mudlump spring on Northeast Pass, col- 
 lected by H. L. Marindin, of U. S. Coast Survey schr. Varina, 
 in February, 1867. 
 
 According to the recollection of one of the crew, this speci- 
 men was taken from the same cone as the following one (No. 
 
 in). 
 
 Water clear, faintly yellowish, about J by bulk of the con- 
 tents ; the rest sandy mud. Turns turbid rapidly on exposure 
 to air. 
 
 * With mixtures of this kind no method but that of evaporation with excess of 
 carbonate of soda will yield anything more than an approximate estimate of the 
 solid residue; involving an amount of labor and care uot always justified by the 
 end in view, when the relative amounts of ingredients can serve to control. The 
 chlorine determination being the most accurate, and almost always in excess of the 
 bases found available to form chlorides, the chloride of sodium, as here recorded, 
 is the calculated amount, as is also the sum of ingredients. 
 
363 
 
 K W. Hilgard Geology of the Delta, 
 
 
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and the Mudlumps of the Mississippi. 
 
 364 
 
 TABLE continued. 
 
 
 Knight's Gas Well. 
 New Orleans. 
 
 Gulf Water. 
 30 miles out. 
 
 Sea Water. 
 Average com- 
 position. 
 
 
 VI. 
 
 VII. 
 
 vm. 
 
 
 a. 
 
 b. 
 
 c. 
 
 a. 
 
 b. 
 
 a. 
 
 b. 
 
 Chloride of sodium, 
 
 0-02310 
 
 37-880 
 
 56-479 
 
 1 6723 
 
 76-870 
 
 2-700 
 
 77-032 
 
 potassium, _ 
 " calcium, 
 
 0-00790 
 0-00060 
 
 12-950 
 0-980 
 
 19-315 
 1467 
 
 0-0357 
 
 1-640 
 
 0-070 
 
 1-997 
 
 " magnesium, - 
 Sulphate of calcium, . . 
 
 0-00630 
 
 10-330 
 
 15-404 
 
 0-2310 
 
 10-620 
 
 0-360 
 
 10-271 
 
 Bromide of magnesium, 
 Sulphate 
 
 0-00300 
 
 4-920 
 
 7-335 
 
 trace. 
 0-1077 
 0-1289 
 
 trace. 
 4-950 
 5-920 
 
 0-002 
 0-140 
 0-230 
 
 0-058 
 3-994 
 6-562 
 
 Carbonate of calcium, _ 0-01120 
 ' magnesium, |0'00760 
 " iron 
 
 18-360 
 12450 
 
 2-130 
 
 
 
 
 0-003 
 
 0-086 
 
 Silica, 
 
 0-00130 
 
 
 
 0-06LOO 
 
 100-000 
 
 100-000 
 
 2-1756 
 
 100-000 
 
 3-505 
 
 100-000 
 
 Specific gravity, . _ 
 
 
 1-01630 
 
 1-0298 
 
 III. Water from faintly active cone on Salt Spring Island, off 
 Northeast Pass ; from crater on west side of island, about ten 
 feet above sea-level. Collected Dec. 2, 1867. 
 
 Water very salty, about f of the whole mass; the rest, a 
 somewhat sandy mud ; color, slightly yellowish ; turns turbid 
 rapidly on exposure to air. 
 
 The larger proportion of water in this specimen, as compared 
 with the preceding, doubtless results from the comparative in- 
 activity of the cone, as compared to that at the time of high 
 water in February. The same fact may account for some of 
 the difference in composition. 
 
 IV. Water from East Crater on Marindin's Lump, Passe a 
 1'Outre. See p. 362. Collected Dec. 2, 1867. 
 
 Forms about f of the bulk in the bottle, the rest is soft 
 clayey mud. Faintly brownish, clear ; becomes brownish tur- 
 bid rapidly on exposure to air. 
 
 V. Water from West Crater on Marindin's Lump, Passe a 
 1' Outre ; same date. 
 
 Water about J of bulk in bottle, the rest clayey mud. Not 
 very salty ; colorless, but turns turbid quickly on exposure to 
 air. 
 
 VI. Water from gas well bored by J. B. Knight, at New Or- 
 leans. Clear, with a little sand at bottom ; taste, faintly brack- 
 ish. 
 
 VII. Water of the Gulf of Mexico, taken from surface 30 
 miles out, southeast from Southwest Pass, Dec. 1867. 
 
 VIII. Average composition of sea-water, according to Keg- 
 nault. 
 
 The general results deducible from the above analyses may 
 be thus stated : 
 
365 E. W. Hilyard Geology of the Delta, 
 
 1. There is a general similarity of composition between the 
 waters of the mudlump springs, indicating their derivation from 
 a common source of supply. But springs on the same lump, as 
 well as the same spring at different times or stages of water, may 
 vary quite sensibly, both in composition and concentration. 
 Their density is generally inferior to that of sea-water, though 
 at times approaching it closely. 
 
 2. There is an obvious approximation of the ratio between 
 the two chief bases sodium and magnesium to that existing 
 in sea- water ; the variations being no greater than are observed 
 in sea-water from different localities. 
 
 3. The most obvious difference is the absence of sulphates, 
 and their partial replacement by chlorides ; also 
 
 4. The presence of large amounts of the carbonates of the 
 earths, as well as of iron, dissolved in carbonic acid. 
 
 5. The amount of potassium salts is decidedly diminished. 
 
 6. Bromids appear to be entirely absent, and are present in 
 traces only, in the water of the Gulf itself 
 
 It is stated that the waters of some of the springs are fresh. 
 I have found all brackish at least, and as the population of the 
 delta is much in the habit of drinking water of questionable 
 freshness, their judgment in the matter may not be the most re- 
 liable. 
 
 As regards the water of the New Orleans well, though at first 
 sight it differs materially from the others, it will be observed 
 that when in C, potassium and sodium chlorides arc classed 
 together, its composition becomes not unlike the mudlump wa- 
 ters. Its great dilution accounts for the excessive proportion of 
 carbonates. 
 
 The Grulf water approaches very closely in its composition 
 to the normal one of sea-water, as given by Eegnault 
 
 Mud from Mudlump Springs. I have already stated, that the 
 stratified material of the mudlumps (whether formed by the ac- 
 tion of mud springs, or bodily upheaved) is free from visible 
 shells or other fossils, save particles of woody matter ; while 
 the amorphous material which forms the surface and generally 
 also the beach, often abounds in such organic remains as are 
 now usually washed ashore, having manifestly been cast up and 
 imbedded in the mud by the waves. 
 
 I have examined microscopically the mud remaining in the 
 specimen bottles after decanting the water for analysis; they 
 were immediately refilled with distilled water, and kept 
 closely stopped until examined. I give below the record of 
 examination of the mud, corresponding to analysis No. Y, 
 from West Crater on Marindin's Lump. Two or three others 
 similarly examined gave a like result. 
 
and the Mudlumps of the Mississippi. 366 
 
 A sample from the general mass shows it to consist mainly 
 of very fine quartz sand, mostly angular, with but a few large, 
 angular grains ; and but little true clay. 
 
 In washing the mass, even the first washings were found to 
 contain but little clay, but chiefly very fine suspended silex. 
 A few ill defined spicules, a Navicula, and bark fibers. 
 
 In the middle portion, spicules a little more abundant. 
 
 In the coarsest portion, much variously colored mica, along 
 with, mostly sharply angular, quartz grains ; numerous particles 
 of water-browned wood ; very distinct spine of a radiate ; sev- 
 eral specimens of Rotalina, and fragments of same as well as 
 Uvigerina, Cristellaria, Amphistegina, and Coccineis ? Also 
 iridescent fragments, showing lines of growth, from the edges 
 of larger bivalves. 
 
 Quartz grains mostly transparent and angular ; some of 
 milky, oil-green, and rose quartz ; these mostly rounded. This 
 sand resembles closely that of the bottom outside Northeast 
 Pass bar, in 40 to 50 feet water ; specimens of which were fur- 
 nished me by the Coast Survey party, under command of Capt. 
 F. V. Webber of the schooner Varina, in 1869. 
 
 Specimens from mudlumps on Southwest Pass show coarser 
 sand, and rather more Foraminifera. 
 
 The character of the materials ejected by the mudlump 
 springs, as determined by the foregoing investigations, may be 
 summed up as follows : 
 
 1. The gas is such as is evolved by vegetable matter in its 
 first stages of decay or lignitization. 
 
 2. The earthy matter contains both river and marine fossils 
 driftwood reduced almost to its cell- elements by maceration and 
 trituration, as well as Foraminifera. Its fineness is such that, 
 before final deposition, it may have been carried out into water 
 of considerable depth. 
 
 3. The mudlump waters appear to be sea-water more or less 
 diluted, and chemically changed under the joint influence of 
 fermenting organic matter, and the more active ingredients of 
 the river deposit, viz: carbonates of lime and magnesia, and 
 oxide of iron. 
 
 The first effect thus produced would probably be the addition 
 of the soluble carbonates of these metals to the solution. But 
 the soluble sulphates could not, in the presence of a soluble 
 iron salt, long resist the reducing influence of decaying organic 
 matter. As usual under such circumstances,* iron pyrites 
 would be formed, withdrawing in the end all the sulphuric acid 
 from the solution, and forming, instead, equivalent amounts of 
 the respective carbonates. The amounts of the chlorides of 
 
 * See Bischoff, Chemische G-eologie, vol. i, p. 559. 
 
367 E. W. Hilgard Geology of the Delta, 
 
 sodium and magnesium originally present would thus, also, be 
 relatively increased ; and this again tallies with the analytical 
 results. Yet while the proportion between these bases is main- 
 tained* the actual replacement of sulphate of magnesium by the 
 chloride constitutes a change not readilv explained. At first 
 sight it would seem that the excess of chlorine belonging to the 
 chlorides of calcium and magnesium must be an outside acces- 
 sion ; but our knowledge of the mutual reactions between the 
 substances here present under strong pressure, is perhaps too 
 fragmentary to justify an assertion on this point. 
 
 The diminution of the potash salts is doubtless referable to 
 their absorption by the clays present, in preference to all other 
 compounds. The filtration of sea- water through soil would, ac- 
 cording to Liebig's experiments, produce a like result. 
 
 Conclusions. In view of the foregoing facts, the explanation 
 of mudlump phenomena suggested, substantially, by Sir Chas. 
 Lyell, seems the only tenable one ; it requires, however, some 
 modification as regards the mass supposed to exert the pressure, 
 and some corollaries as to the mode of action. 
 
 I have before suggested, that the rapid protrusion of the 
 mouths of the river into the Gulf, in advance of the body of 
 the delta, is owing to the shelf of " blue clay bottom " extending, 
 at a comparatively slight depth below the sea-level, and with a 
 gentle slope, far out into the Gulf. It is upon this impervious 
 clay that the present bars are based ; and upon it, in advance 
 of the bar, will be deposited the finest of the river mud, at a 
 depth at which, perhaps, the sea-water is at the time quite 
 undiluted, and fully adapted to marine life ; which will there- 
 fore deposit its vestiges in it, associated with the finest particles 
 of driftwood, etc. Mud thus deposited may remain unconsoli- 
 dated for a great length of time, unless the water be drained 
 off or through it by some means. 
 
 The bar, in its annual advance of about 338 feet, will cover 
 over this liquid mud stratum, exerting a pressure much greater 
 than that of the sea ; and were it resting on a pervious bottom, 
 the liquid mud would doubtless soon be consolidated into a 
 sheet of clay. As it is, the tendency will be to squeeze it 
 from under the crest of the bar, both seaward and landward. 
 The very gradual seaward slope of the bar will render the 
 movement in that direction a very slow one, under a greater 
 depth of water and heavy frictional resistence. Not so on the 
 land side, where, as the bar advances, the superincumbent pres- 
 sure is measurably relieved by the erosion of a channel by the 
 
 * The ratio is for 
 
 Atlantic sea-water, 30 '5 : 3'0 
 
 German Ocean water, 30 - 5 : 4'0 
 
 Average sea " 30'5 : 3*4 
 
 Marindin's Lump water, 30 - 5 : 2'8 (min.) 
 Southwest Mudlump, 30'5 : 3*56 (max.) 
 
 Average of those analyzed, 30*5 : 31 
 
and the Mudlumps of the Mississippi. 368 
 
 current, which, especially in flood time, carries much of the 
 bottom deposit bodily over the bar and drops it to seaward.* 
 At weak points of the bottom inside the bar, therefore, the 
 upward pressure of the mud may cause a bulging up at least 
 to the level of the bar-crest, and, perhaps, taking into account 
 the difference in the specific gravities of the comparatively 
 solid bar and liquid mud, even as far as the surface of the 
 water. But this, considering the question as coming within 
 the domain of liquid statics, would seem to be about the ex- 
 treme limit to which the bottom itself could be brought up. 
 
 I have found the specific gravity of the mud .flowing from 
 the West Crater on Marindin's Lump, to be about 1 '25 ; that from 
 the cone on Salt Spring Island, on Northeast Pass, 1'30 ; while 
 that of bar deposit from the crest of the Northeast bar, wet as 
 brought up by the lead, was 175. The heights of communi- 
 cating columns of these substances, if sensibly liquid, should 
 be as 5 to 7 ; but this ratio could apply, in the case of upheaval, 
 only so far as the difference of level between the bar and the 
 upheaved bottom is concerned, since the latter must be pre- 
 sumed to be similar to the bar in its materials and structure. 
 Should the current, however, continue to denude the crest of 
 the upheaved mass, the rising would continue and the semi-fluid 
 mud might finally break through, forming a mudspring, the 
 height of whose vent above its source might finally increase to 
 the extent corresponding to the difference of specific gravity. 
 
 The craters of mudlumps have been sounded to the depth of 
 24 feet, but no precautions were used to insure reaching the 
 actual maximum depth. In the borings made for the founda- 
 tion of a lighthouse on the Southwest Pass, by Mr. A. Palms 
 (the record of which was courteously forwarded to me by the 
 Engineer Dept), a stratum so soft that the augur sank in 'it by 
 its own weight, was met with at 58 ft., after striking, at 56 ft. 
 a stream of water which "filled the pipe." 
 
 If then, the mud stratum, lying, say 60 ft., below the surface, 
 be pressed by a column of deposit of 1*75 sp. gr., mud of 
 sp. gr. 1-25 could thereby be raised 24 feet above the top of 
 the pressing column ; and this, considering the average depth 
 of water on the bar, would account for the greatest heights to 
 which cones are built up off the mouths. 
 
 But this is a close calculation, even if the data upon which 
 it is based be deemed admissible in the form I have given them ; 
 and the frequency and energy with which the upheaving force 
 acts, coupled with the fact that when a mudlump rises in the 
 channel, so far from suffering denudation to the extent re- 
 quired for the breaking through of the mud, it more generally 
 causes a silting up of the channel : seems to me to indicate that 
 
 * Humphreys and Abbot's Kept., p. 446. 
 
369 E. W. Hilgard Geology of the Delta, etc. 
 
 a stronger force, less delicately balanced than the equilibrium 
 of the bar, is " at the bottom " of the whole phenomenon. 
 
 This force, I think, is to be sought in the constantly increas- 
 ing weight of the alluvial area above the mouths, which, itself 
 possessing a series of mudlump vents at one time, must yet be 
 resting in a great measure upon the still unexhausted mud 
 stratum ; as is proven by the existence of active lumps in the 
 marshes, even though the increased resistance of superincum- 
 bent deposit as well as matted vegetation must render their 
 occurrence there a rarity. There must still be a communica- 
 tion of liquid pressure "between the older and newer portions 
 of the modern delta ; and this point is especially strengthened 
 by the fact that a high stage of water in the river, which does 
 not sensibly affect the depth on the bars, yet exerts a decided 
 influence on mudlump activity. The river not only overflows 
 the marshes, but loads them with additional sediment; and 
 doubtless the increased hydrostatic pressure stops many a vent 
 of gas, mud or water, which ordinarily discharges into the 
 river's bed.* 
 
 Morgan's Lump, in the marsh of Southwest Pass, and 
 Marindin's Lump on Passe a 1'Outre, are now known to have 
 been in undiminished activity for twenty-five years at least. 
 Since that time the bars have moved gulfward a mile and a 
 half; and one would think that, if the activity of the cones 
 depended upon them alone, a notable difference ought to have 
 been observed. But if the main force is a vis a tergo, while 
 the bar serves mainly to prevent the escape of the mud to 
 seaward, there is good cause for the secular persistence of vents 
 that have escaped mechanical obstruction. 
 
 How far above the present mouths the head of pressure may 
 extend, I do not pretend to conjecture. The borings at New 
 Orleans seem to indicate that the mud stratum originally 
 existed there also, but it would be extravagant to suppose that 
 such pressure as that exerted in the gas wells of that city, could 
 now be felt a hundred miles below. Yet it seems not at all 
 unlikely, that the weight which steadily forces up the liquid 
 mud to the top of Morgan's Lump, seven miles above the 
 mouth, may, in part, be furnished by the enormous mass of 
 vegetation which annually develops in the marshes, willow 
 battures, and perhaps even cypress swamps above. Nor is the 
 effect of gaseous pressure resulting from the constantly pro 
 gressing decay of organic matter to be overlooked, although I 
 doubt that this cause plays, ordinarily, anything more than a 
 very subordinate part. 
 
 * I give on the plate (page 358) an ideal section, illustrating this explanation of 
 the "origin of mudlumps." 
 
and the Mudlumps of the Mississippi. 435 
 
 Future observations, systematically carried out, will doubtless 
 solve a good many of the questions here mooted ; and though 
 they may not lead to the suggestion of any means whereby 
 the k ' evil geniuses of the Passes " may at present be conjured, 
 a more precise knowledge of data, as well as of the statics and 
 dynamics of mud, may enable us to predict at what point of 
 advance of the mouths into the deeper water of the Gulf, their 
 formation must cease. The Southwest Pass appears to be 
 nearest that consummation devoutly to be .wished ; and were 
 the closing of the other outlets practicable, the advance of the 
 Southwest bar might become so rapid, as to let the youngest 
 of the living generation witness a diminution of mudlurnp 
 upheaval. Ultimately, the mouth might thus become similar 
 to those of the Orinoco and Amazon ; but until then, ceaseless 
 activity of the river in the formation of bars and mudlumps 
 must, in the interest of navigation and commerce, be met 
 by an equally ceaseless and diligent effort for their removal 
 from the channel. For while a concentration of the river cur- 
 rent might possibly be made to maintain the needful depth upon 
 the bars, its utmost erosive energy will be powerless against the 
 tough, inert masses of the mudlumps. 
 
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