SAINT LOUIS, August 10, 1861.

Major General J. C. FREMONT, Saint Louis.

DEAR SIR: At the present time the counties of Monroe, Ralls, Marion and Shelby are infested by bands of armed men encamped in different places and frequently changing their place of encampment. It is believed that at the present time a large majority of the people of Monroe and Ralls counties favor secession. In Marion and Shelby the majority is not so large. I think the number in camp in these counties must be between 1,000 and 2,000 and such is the communication kept up between them that if they desired to concentrate a force at any point I have no doubt they could bring out over 2,000 at short notice including those who are usually at home at their work. In addition to the rifles and shotguns of the country they have some muskets with bayonets; these are said to be about 400 in number. They also have two cannon, 6 and 9 pounders, made at Hannibal. It is said they have others taken from the Liberty Arsenal. I think they have others.

These men are exceedingly bitter in their feelings of hostility and have been led on until many of them are fit for any deed. Usually they are ignorant; they are fed on falsehood and are encouraged in their course in the strong belief that Jackson is soon to reinstate himself as governor of the State. To this the defeat at Manassas and the invasion of the State from the South-aided by false statements, such as that Bird's Point has been taken and that Lyon and Sigel have been cut up-gives great encouragement.

It must be confessed that there have been many aggravating causes to produce this state of things. You already know the many depredations committed by the soldiery. Perhaps this has not been the worst. Frightful stories as to what the soldiers would do if they came into the State preceded them on their approach to a place. Many were ready to run from fright. It occurred to my knowledge in a good many cases where men thus ran and did not obey the order to halt, which very likely they did not understand if they heard; they were fired upon- not single shots but volleys-in the presence but without the command of officers. Whether any were killed in this way I do not know. It has been reported to me that soldiers have repeatedly fired from trains at quiet, peaceable citizens. I believe this though I have not seen it. Very many have been arrested without any cause except that they were reported secessionists; and not only this but indignities have been put upon them such as requiring them to "mark time," dig ditches and sink-holes for filth. The present week Mr. McAfee, speaker of the last house of representatives, was arrested and required by General Hurlbut to dig trenches in the hot sun as I was told all day. Hurlbut himself told me he set him at it. McAfee is no doubt a very bad and dangerous man; still it was admitted that it was very doubtful if any charge could be maintained against him. If he is now let go for want of cause to hold him I fear he will be able to do us much more hurt than heretofore.

Now, sir, when these facts which are bad enough are greatly exaggerated by crafty men they have led many especially young men into a bad cause from really noble and generous impulses. When once they are in and have committed the overt act it is hard to get them out. These things have tended greatly to weaken the Union cause and in the State where I am acquainted there are far less Union men than two months since.