Page images
PDF
EPUB

all obstructions, shall have been formed entirely across the tidal bar, from the secondary base surveyed by us to the open gulf.

“11th. In general you are desired to ascertain and report all perceptible changes in the bed of the Pass below the secondary base above mentioned, and especially in the positions, widths, and depths of the channels below that line.

“12th. You are authorized to purchase such articles of stationery, and to hire such boats, hands, tools, &c., as are necessary to the performance of the duties assigned you, with the understanding that vouchers duly executed in duplicates must be taken and rendered for all such expenditures.

“13th. Preparations having been made for the adjustment of all accounts likely to be incurred during the current quarter, you are desired to forward to the undersigned monthly estimates for each month of the succeeding quarter, in order that proper remittances may be forwarded for defraying the expenses of each month of the quarter last mentioned, (third quarter of 1857.)

“14th. Your attention is particularly requested to the observance of each and every item of the foregoing instructions, by means of which I shall be kept adequately apprised of the progress of the work committed to your supervision.

“In conclusion, I would observe that your experience as an engineer, and your familiarity with nautical affairs, inspire the belief that you will be able to discharge the duties confided to your charge in a manner creditable to yourself and beneficial to the public service. “Respectfully, sir, your obedient servant,

"S. H. LONG,

" Lieutenant Colonel T. E. Captain WM. JOHNSON, Southwest Pass.Having accomplished my inspections, triangulations, and other arrangements, as above, I communicated the following notice to Messrs. Craig & Rightor, the contractors for the improvement of the Southwest Pass :

“SOUTHWEST Pass, April 20, 1857. “GENTLEMEN : The bearer, Captain Wm. Johnson, having been employed to inspect your work for the improvement of the Southwest Pass, and to report its progress, and the effects produced thereby, in so far as relates to the removal of obstructions from the main navigable channel and to the formation of a new channel, 300 feet wide and 20 feet deep, as stipulated by contract, you are desired to receive him in the capacity above intimated during my absence, and to regard him as my agent and representative in all matters relating to the fulfilment of your contract till I return again to the Pass.

" Whenever the work shall have been so far advanced as to enable you with certainty, and to the satisfaction of Captain Johnson, to fix à date at which it will be ready for final inspection and acceptance, agreeably to the contract, you are desired to apprise him of such date, that I may be seasonably notified thereof by him and return again to

this place for a personal inspection and acceptance of the work, if executed in conformity to the terms of the contract. Respectfully, gentlemen, your obedient servant,

“S. H. LONG,

Lt. Col. Corps Top. Engs. " Messrs. CRAIG & RIGHTOR,

Contractors, Southwest Pass."

In conclusion, I take leave to present a further exposition of my views in relation to the formation and character of the tidal bar, and, for this purpose, shall adopt the following hypothesis, based on the present aspect of things and upon occurrences known to have taken place in former times.

The tidal bar is known to be composed of sedimentary matter brought down and deposited by the waters of the Mississippi, and consisting of earthy and vegetable materials, exceedingly comminuted, unctuous, and adhesive.

The points at which the most copious depositions have been made are those at which the currents of the river are merged into the still waters of the gulf. The deposits brought down in each successive year are made in advance of those of the preceding year, thus contributing to the annual advancement of the tidal bar into the gulf.

The deposits, when first made, as before intimated, are fine, unctuous, and impalpable, and constitute a semi-liquid mass of earthy, mineral, and vegetable particles, which, when brought into contact and combination with the salt water of the gulf, are subjected to chemical and mechanical changes, the former of which have never been adequately investigated or explained.

With respect to the mechanical changes, or those produced by the combined agency of the river currents and gulf billows, they may be accounted for in part, if not altogether, in the following manner :

The annual amount of deposit is distributed along the outer margin of the tidal bar in accumulations more or less abundant, according to the quanties of fresh muddy water conveyed across the crest line of the bar at or near the points of deposition.

The deposits having been made, the gulf billows begin to exercise a mechanical action upon them, which results in the formation of a crust or indurated covering upon their surface, which continues to increase in thickness and solidity till, at length, the covering of the mass becomes firm and unyielding, and, at the same time, impervious to the gasses that are formed by the decomposition of the vegetable matter contained in the mass of deposits confined beneath the crust or covering.

In this way annual deposits, accumulations, &c., are constantly occurring, in proof of which, borings through the alluvial formations of the Mississippi delta, from its surface to great depths below, have developed a succession of stratifications of mud and indurated clay of different degrees of hardness, alternating with each other to the full extent hitherto penetrated.

The decomposition of the vegetable matter probably occurs annually, or, perhaps, less frequently after the subsidence of the spring

[ocr errors]

The gasses

and summer floods of the Mississippi, when the waters, &c., adjacent to the gulf shall have been brought to such a degree of temperature as is necessary to produce fermentation and putrefaction. At this and all higher degrees of temperature the process of decomposition is carried on more or less rapidly, attended by the production of gasses, which, in combination with the soft mud overlaid by the crust or indurated covering before mentioned, gives to the underlying mass a specific gravity materially less than that of the water, deposits, &c., lying above the crust. Hence the crust must yield at its weaker or thinner points, an uprising of the same ensues, and those hitherto anomalous productions called “ mud lumps” ensue.

The uprising of the indurated covering continues until the coverings crack open at their highest points, and the gasses gradually escape through the fissures, carrying with them small streams of water, exceedingly turbid, and of saline and bitter taste. thus escaping are invariably inflammable, like the bubbles arising from the bottoms of fens or stagnant pools.

The numerous mud lumps that have come under my personal observation have invariably presented a covering of stiff adhesive clay, rent and divided by fissures on almost every portion of their surface. They assume all possible forms; those of the smaller dimensions being somewhat conical in their form, while those of the larger sizes present themselves iu the shapes of ridges, some of them nearly straight, while others are curved and recurved, presenting very irregular and ragged outlines. Some of them present orifices like craters, through which the gasses and salt springs escape, while others present no other openings but prolonged fissures, through which their imprisoned air, mud, and water are set at liberty.

It is true of almost every mud lump, however minute or spacious, and whether above or below the surface, that a pole or stake driven through their indurated covering, enters into a soft yielding mud, and may readily be thrust downwards to a great depth with very little resistance.

Mud lumps of the character above described are nowhere to be met with except in the vicinity of the inner margin of the tidal bar, of which last, as also of the swamps in its rear, they constitute not only the substratum, but the entire superstructure of mud, &c., erected thereon.

S. H. LONG,
Lieutenant Colonel Topographical Engineers.

No. 11.

Colonel Long to Colonel Abert.

LOUISVILLE, June 20, 1857. SIR: Sundry letters have been received from Captain Wm. Johnson in relation to the operations of Messrs. Craig & Rightor at the Southwest Pass, and to the changes producedt hereby in the channels

across the tidal bar during the month of May last. Hence it appears that early in that month a strong gale from the southeast prevailed for several days, and contributed to the rupture of the upper portion of the line of piles, through a distance of about 800 feet; also, that at the end of the month the rupture had been repaired, and that the contractors continued sanguine in the belief that their enterprise would be crowned with ultimate success.

At the date last mentioned, (May 31,) the contractors had succeeded in repairing all defects in the line of piles, and in extending the same to an aggregate length of about 3,000 feet. The work thus extended remained stable and unaffected by storms at that date; no violent gales having occurred since the protracted gale above noticed.

From the observations of Captain Johnson, it also appears that no perceptible changes have been produced in the bed of the Pass, except a slightly accelerated current along the upper side of the line of piles, the depths of the soundings across the Pass, as also in the channels across the tidal bar remaining unchanged.

Captain Johnson has been directed to continue his observations in the manner prescribed in my instructions of the 18th of April last, a copy of which has been sent to the bureau, and to keep me apprised of the results produced by the operations of the contractors, while I hold myself in readiness to revisit the work whenever it shall appear that the contractors shall have got it ready for final inspection.

In addition to the documentary information received from and through the bureau on the subject of the passes, I have succeeded in obtaining the manuscript reports of Captain Talcott, and his associates, Messrs. Lidell & Meade, on the same subject, and have caused the same to be entered among the records appertaining to the improvement of the mouths of the Mississippi.

S. H. LONG,
Lieutenant Colonel Topographical Engineers.

No. 12.

Colonel Long to Colonel Abert.

LOUISVILLE, July 30, 1857. SIR: Agreeably to a report of Captain Johnson, received on the 28th instant, it appears that the operations of Messrs. Craig & Rightor at the Southwest Pass, during the month of June last, have been attended with serious discouragements and doubtful success. The line of piles has been extended southwestwardly only about 300 feet during the month, while the line, through a distance of about 100 feet of its northeastern portion, has been disrupted and driven from its moorings.

Hence the extent of the line still remaining on the 30th June is only about 3,200 feet, while as yet it appears that nothing had been done towards the formation of a counter line of piles on the northwesterly side of the Pass.

Boisterous weather had prevailed during most of the month, by which not only the progress of the work had been much retarded, but some four or five of the signals for the triangulations had been carried away.

With respect to the improvement of the channel, no favorable changes are yet perceptible, and doubts are still entertained as to the efficiency of the mode of improvement adopted by the contractors under the sanction of the War Department.

S. H. LONG, Lieutenant Colonel Topographical Engineers.

No. 13.

Colonel Long to Colonel Abert.

LOUISVILLE, August 20, 1857. Sir: Agreeably to a report of Captain Johnson, dated on the 31st ultimo and this day received, it appears that the work of Messrs. Craig & Rightor had advanced rather tardily during the month of July last, and that the benefits resulting from it continued doubtful at that date.

The line of piles had been prolonged southwestwardly towards the tidal bar, with a deflection of about five degrees to the right, through a distance of about 1,100 feet, making the entire length formed at the close of the month about 4,300 feet.

From the soundings on the lines of the triangulations it does not appear that either the depth or width of the main navigable channels has been increased in consequence of the work that has been done under the contract, except in so far as relates to a perceptible divergence of the outflows of the water across the tidal bar from the left to the right or northward, which appears to have been occasioned by the line of piles.

The weather appears to have been less boisterous and more favorable for the prosecution of the work during the month of July than it had been during the preceding month.

S. H. LONG.

No. 14.

M. P. Breckinridge to Col. Long.

LOUISVILLE, September 1, 1857. Sir: In compliance with your order of August 12th, enclosing a copy of the queries from the topographical bureau of April 24, 1856, and other papers, requiring of me a report upon western commerce and the steamboat disasters of the western rivers for the fiscal year

« ՆախորդըՇարունակել »