river, from Cairo to the Gulf, as myself, and that this very subject of cut-offs was one of those carefully examined by me.
There are certain general facts bearing upon the practicability of making the cut-off referred to, the application of which is simple. Those facts are: First, that the bed of the Mississippi is composed of a hard, tenacious blue clay, not an alluvial deposit, which even the strong currents of that river wear so slowly as seemingly to produce no effect upon. Next, that the alluvial soil found upon the surface in the vicinity of Vicksburg does not exceed 30 or 40 feet in thickness, and is composed of alternate layers of loam and clay of variable thickness, the clay having some of the tenacity of that forming the bed of the river, though not of the same age. Next, that the bottom of the Mississippi is always found in one of those beds of clay, for the current cuts immediately through the layers of sand.
This being premised, the first thing to be done to ascertain the practicability of making a cut-off, and the depth to which a channel must be excavated to bring the erosive and wearing power of the current into action, is to make borings on the site of the proposed cut-off, so as to ascertain precisely the number and thickness of the different layers of clay and loam and sand. In low water the strata on the river banks in the vicinity should be, and I presume were, in the case referred to, examined. In some localities there is but one stratum of clay of no very great thickness to be cut through to complete the cut-off. Such was probably the condition existing at the site of the American Bend cut-off, a natural cut-off made a few years ago. The river above and below approached so closely in this case that it is probable the layers of loose sand of the old formation (not the alluvial) washed through and broke up the superincumbent layers of more tenacious soil. Even the loam of the Mississippi alluvion has considerable tenacity. In other localities the cut-off is impracticable, owing to the number and thickness of the layers of clay. The boring is a simple operation. The first beds of clay, those nearest the surface, must be cut through (blasting was found to aid greatly in making the Red River cut-off), so that the current may have the loam and sands only to impinge against and wear on.
Simple as this matter is, I know that it was not in the least understood when I began the investigation upon the Mississippi River, and the facts are not to be found in any other work than the report upon the Delta. Few and simple as those facts are, the labor required to collect and digest them was by no means small.
Perhaps submitting this letter to the honorable Secretary of War may be the readiest mode of bringing the facts to the notice of the commander of the army, now before Vicksburg, if he has not already been made acquainted with them.
Very respectfully, your obedient servant,
A. A. HUMPHREYS,
Brigadier General Comdg. Div., Major Top. Engrs.
HEADQUARTERS THIRTEENTH ARMY CORPS,
Before Vicksburg, February 14, 1863.
Lieutenant Colonel John A. RAWLINS,
COLONEL: I have the honor to transmit, for the information of the major-general commanding, by order of Major-General McClernand,