HEADQUARTES DEPARTMENT OF THE MISSOURI,

Saint Louis, December 19, 1861.

Major General GEORGE B. McCLELLAN,

General-in-Chief of the Army, Washington City:

GENERAL: The expeditions of Generals Prentiss and McKean through the counties norh of the Missouri River have been entirely successful. Every insurgent organization there has now been broken up, and I think it will be safe in a short time to withdraw a portion of the troops from that district. At Glasgow our troops yesterday captured about two tons of powder, which it is said was purchased in Chicago and brough in on the Saint Joseph and Hannibal Railroad among dry goods and in demijohns marked " brandy." It was carried through in small quantities by rebel country merchants and farmers and collected at Glasgow, to be thence forwarded to General Price. Colonle Marshall's expedition to Arrow Rock and Waverly prevented its crossing the river, and it consequently fell into the hands of our troops on the north side.

General Pope's party intercepted a portion of the insurgents who crossed at Lexington. He attacked their camp of 2, 200 near Shawnee Mound, between Warrensburg and Clinton, and completely scattered them, capturing their tents, wagons, horses, baggage, &c., and taking 150 prisoners. If General Prentiss had kept me advised of his movements, as I had directed, I would have taken the whole party. Our forces at Sedalia were for several days ready to move at a moment's warning, and only waiting to hear where Prentiss was. On the whole, however, the result is satisfactory. We have within the last two weeks taken about 500 prisoners, one piece of artillery, a number of arms, wagons, horses, and a considerable amount of ammunition and clothing; and, what is of most importance, we have pretty effectually crushed out the insurrection along the Missouri River, upon which Price placed his main reliance for recruits and clothing. His army is pretty well supplied with provisions and arms, but is greatly in want of clothing and shoes. He has a large park of artillery, and from all I can learn will make a good fight. He and many of his men are virtually outlaws, and, it is said, are determined to win a victory or die. Another retreat would effectually ruin his cause in this State. I had hoped to draw him north to the Missouri River, so that I could effectually cut off his retreat, but his main force has not yet crossed the Osage, and the columns which he advanced on Clinton and Warsaw have been withdrawn. In that position I cannot cut him off by moving from Rolla, as our would be completted to cross the Osage Mountains over narrow and difficult roads, which would make our movements necessarily slow, affording him plenty of time to fall back. It is therefore possible that I may be compelled to attack him in front. The ravages committed by his troops may made it advisable to move against him soon, notwithstanding the great inconveniences of a winter campaign in this climate, which is very much mover unfavorable than that of Washington. The roads, too, will probably be almost impassable. I hope, however, to be able to overcome these obstacles. At any rate, I shall not hesitate to act as soon as I can get the troops in anything like organization and discipline.

The conduct of the columns sent through the river counties forms a striking contrast to that of previous expeditions. Citizens from these counties assure me that they have all acted with perfect propriety, committing no depredation, and producing a most favorable impression. This certainly indicates a vast improvement in discipline. By a few