General Rosecrans. The extent of our defeat and loss is not known here. General Rosecrans will require all the assistance you can give him to hold Chattanooga.

H. W. HALLECK,

General-in-Chief.

MORRISTOWN, TENN., September 21, 1863-11 a.m.

(Received 12 m. 22nd .)

Major General H. W. HALLECK,

General-in-Chief:

Your dispatch of 20th received. Before I knew of the necessity of sending immediate assistance to General Rosecrans I had sent a considerable portion of my force to capture or drive out a large force of the enemy under General Sam. Jones, stationed on the road from Bristol to Jonesborough, which amounts to at least 6,000 men. I had an ample force on the way to have in all probability accomplished this object quickly, when the urgent dispatches of General Rosecrans and yourself caused me to send General White's division and Colonel Wolford's brigade of cavalry, with orders to move as rapidly as possible until they joined General Rosecrans' left flank.

Colonel Byrd was ordered to Athens with his cavalry brigade soon after we occupied Knoxville, and was afterward ordered to send a portion of his force to Cleveland, which he did, but was driven out of there three days go. Colonel Wolford's joining him will increase the force to over 3,000 men, but they are now ordered to move down at once and attack Forrest, who is said to be occupying Cleveland.

There is great difficulty in crossing the force over the Holston, as we have no pontoon-bridge ready yet. One is being built at Loudon, which will be finished in a day or two, after which troops can cross and recross rapidly. General White's infantry division will follow down to support the cavalry as rapidly as possible. The advance of the Ninth Army Corps will be here to-night, and will be at once put in motion down the road. The force under Jones at Zollicoffer is over 6,000, and I have but about that number opposed to him. Nothing but provost guards are left at Knoxville, Loudon, and this place, but all the force at Cumberland Gap will be brought down.

I have not left a single guard on my lines. When you remember the size of our forces, the amount of work which it has had to do, and the length of line occupied, you will not be surprised that I have not helped General Rosecrans; more particularly as I was fully impressed with the truth of the statement that Bragg was in full retreat. It does not seem possible for me to successfully withdraw my forces from the presence of Jones if he should be beaten back or captured; yet, upon the receipt of your dispatch if it were possible to get our force from there down make the attempt, and shall, at the risk of being too late, order every available man in that direction. I am sure that I am disposed to give him every possible assistance. I sincerely hope that he will be able to at least check the enemy for seven or eight days, within which time I shall be able to make considerable diversion in his favor., I hope that my action will meet with the approval of the Department.

A. E. BURNSIDE,

Major-General.