HEADQUARTERS DEPARTMENT OF THE OHIO,
Camp at Beverly, Va., July 12 - 8 p. m.

COLONEL: I have the honor to inform you that army under my command has gained a decisive victory, which seems to have accomplished the objects of my march. I turned the enemy's very strong entrenchments on Rich Mountain yesterday with General Rocescrans' brigade of four regiments and one company of cavalry. He had a spirited action with a large party of the enemy (who had two guns) on the summit of the mountain, captured both guns, and killed a large number of the enemy.

This morning, as we were in the act of moving twelve guns into a position commanding the enemy's entrenchments by a road cut last evening, it was ascertained that he had left in the utmost confusion. We have all his guns (six in number), all his wagons, tents, &c. The number if killed, wounded, and prisoners is large. Our loss very small - about 11 killed and 35 wounded.

I advanced at once on Beverly, and occupied it with the least possible delay, thus cutting off Garnett's retreat on Huttonsville, and forcing him to take the Leedsville and Saint George road. I have ordered General Morris to press him closely, and have also given instruction by telegraph which will throw from 5,000 to 6,000 men and four guns in his front, so that there is good reason to hope that we may yet capture him. I shall move on Huttonsville to-morrow morning, and endeavor to seize the Cheat Mountain pass before the enemy can occupy it in strength. With that pass seized, the position on Cheat River (near Rowlesburg) strongly occupied, and the Gauley Bridge held, as it probably is by this time, by General Cox, I think we shall have placed the occupation of Western Virginia on a safe basis. After reaching Huttonsville I can determine better than I now can whether it will be advisable to occupy the Greenbrier Valley at present.

I hope to able to give you to-morrow full details of the transactions of the last few days. I advanced so rapidly to this place that it is not now in my power. We occupy here a very strong defensive position. We are constantly picking up more prisoners.

I am, colonel, very respectfully, your obedient servant,

GEO. B. MCCLELLAN,

Major-General, U. S. Army.

Colonel E. D. TOWNSEND,

Assistant Adjutant-General, Washington.

Captured official papers show Garnett's force to have been 10,000 men. Please give instructions as to the disposition to be made of the prisoners of war. I shall for the present order them to Columbus, Ohio. We have several officers among them Dr. Taylor, formerly of the Medical Corps, U. S. Army.

G. B. MCC.

BEVERLY, VA., July 13, 1861.

Colonel E. D. TOWNSEND:

Success of to-day is all that I could desire. We Capture six brass cannon, of which one is rifled, all their camp equipage and transportation, even to his tents. The number of tents will probably reached two hundred, and more than sixty wagons. Their killed and wounded will amount to fully one hundred and fifty; at last one hundred prisoners, and more coming in constantly. I know already of ten officers killed