POINT OF ROCKS, Sunday, October 6, 1861.

Colonel MARCY:

I arrived this morning at this point, traveling all night from Williamsport. It is my duty to inform you that the Potomac is not properly guarded there, as I came across myself, a little below, in a small boat piloted by a negro, unchallenged.

The movements of troops in Virginia are numerous and uncertain. They are expecting an attack and scarcely know where to look for it. They have been strengthening a place called Brentsville, which is approached from Occoquan Creek and also Dumfries.

The arrangements of Beauregard have been materially interfered with by Johnston and also by the authorities in Richmond. By the orders of the latter, four regiments since last Tuesday have left by rail for Tennessee, and the cars up to last night had not returned; and by Jonston's [order] four regiment (about 2,500 men) have been stationed in the neighborhood of Newtown and Berryville, in the neighborhood of Winchester. Strong pickets are out north and east of these places. I discovered the sole object of this expedition. It has been represented to Johnston that as soon as the river is low enough 1,200 men could cross at Williamsport, take the First Maryland Regiment prisoners, and obtain supplies of salt and other necessaries for which they are in distress. I feel sure this will be attempt unless provided against.

A council of war was held at Manassas on Thursday and immediately afterwards two general officers left for Richmond. The feeling is prevalent among the troops, and it is said to be shard in by Beauregard, that the present rebel army of the Potomac is not large enough to cope with General McClellan's forces, whilst Johnston prates of their "invincibility." The forces as Leesburg have been kept up to nearly 27,000. The troops sent north were taken from positions near Middleburg and Falls Church.

I believe there is no intention to cross the river except on the Upper Potomac, where they make sure they could recross, before being interrupted. There is very little ammunition at Leesburg. A messenger was sent there for some for the troops near Winchester; he was told to go farther south, as they had only 24 rounds for each man.

Pardon my suggesting that if the national army advance shortly, and Occoquan Creek could be threatened at the same moment, there would be a general falling back upon Manassas, and that by a prompt movement via Falls Church, and a simultaneous one on the part of General Stone, the whole force at Leesburg might be captured.

While the rebels are less hopeful about Washington, they are very jubilant at the state of things in Missouri. I was in Richmond one whole day, and whilst there was informed that a message had been received by President Davis from General Price, stating that if 6,000 disciplined troops could be sent immediately, he would establish his headquarters in Saint Louis within ten days.

At my request Colonel Geary telegraphed to you immediately upon my arrival to have a man named Larmour, at Baltimore, arrested. He is expected at Manassas again in a week. He was several times taken letters and information, and took letters there with important information just previous to McDowell's advance.

There are two men now in Baltimore or Washington who have left Manassas on "spying" expeditions; one is named Maddox; he belongs to Loudon County; was once a medical student at Jefferson College, Philadelphia; said to be a very smart fellow. I should have come to Washington, but these men may have seen me when there ten days ago;