This expedition consisted of five infantry regiments- the Thirty- third and Ninety- ninth Illinois, the Eighth and Eighteenth Indiana, and the Twenty- third Iowa- not averaging over 500 men each. Accompanying these troops there were, as near as I could learn, eight pieces of artillery. This brigade was commanded by Colonel [Henry D.] Washburn, acting brigadier- general.

The expedition was directed by General [Cadwallader C.] Washburn, who commands, I understand, one division of the Thirteenth Army Corps, U. S. Army.

General Washburn marched on Saluria the same evening that I landed on the island, the 22nd instant. About two hours after my landing, I was taken by Colonel [Nathen A. M.] Dudley, inspector- general of the Thirteenth Army Corps, U. S. Army, to Mustang Island, when the colonel informed me I would be detained there three or four days for prudential motives. Quarters were assigned me on one of their transports.

I was detained within the Federal lines four days, during which time, being restricted only by the limits of the transport on which I was quartered, I had an opportunity of gaining considerable information respecting General Banks' expedition against our State.

Until the 25th, the five regiments that marched against Saluria and two companies of negroes that remained on Mustang [Island] comprised General Banks' entire command this side of Brownsville. On the evening of the 25th, however, a vessel arrived from New Orleans, I think, with two regiments of infantry- the Twenty- first and Twenty- second Iowa; they also averaged only about 500 men each. This comprised General Banks' entire command on the islands of mustang, Saint Joseph, and Matagorda. When I was released, on the evening of the 26th instant, he had outside four heavy ships of war- the McClellan, his flag ship; the Monongahela, their heaviest armed ship; the Granite City, and the Thomas Scott. Inside of the bar, there were six transports drawing under 8 feet water, all armed but one, their armament consisting of 12 pounder Dahlgren howitzers and 20 pounder Parrott guns. There were four of each kind; two of the Parrott guns, however, were in battery on land, two small earthworks having been thrown up as soon as the island was captured. One of the transports was the Matamoras, from the Rio Grande, one of the boats owned by Messrs. King &Kennedy, of Brownsville; all of their boats excepting one are in the hands of the Federals. The Matamoras is still commanded by her same captain, Dalzell.

While writing here of the Rio Grande, I will mention that the chief of General Banks' staff told me several times that all cotton in Matamoras would be seized and held by the Federal authorities.

As nearly as I could judge from my conversations with General Banks' staff officers and officers of the line, the primary object of this expedition is to completely blockade our coast by capturing our passes and fortifying them. After this is accomplished, they expect, with the assistance of their forces in Arkansas and Louisiana, to invade the State and hold it, as they do other portions of our Confederacy. I was told by one of the above officers that they expected more troops daily- a sufficient number to defeat the army of 20,000 that they understood we had in Texas.

Believing this to be about the amount of information I gained during the four days that I was detained within the Federal lines, I have the honor to be, very respectfully, your obedient servant,

WALTER L. MANN,

First Lieutenant, and Acting Assistant Adjutant General.