HEADQUARTERS DEPARTMENT OF CALIFORNIA, San Francisco, August 11, 1865.

Lieutenant General U. S. GRANT,
Commanding the Army of the United States, Washington, D. C.:

GENERAL: Owing to the interruption of the telegraph line between this and the East, and, further to my absence from headquarters on a tour of inspection beyond the Sierra Nevada to Owens River Valley, your telegram of the 10th ultimo asking if I need troops for Arizona and if so what number and kind should be sent, was not received by me until the latter part of July, since when I have been waiting from day to day for the telegraph to work to send an answer. But as there seems now no prospect of the line being in order for an indefinite time to come I answer this by the next steamer, sending a duplicate to take its chances overland. In the last dispatches received from him, Brigadier-General Mason, commanding the District of Arizona, writes as follows:

I would respectfully request that, if possible, two additional regiments of infantry be sent to this Territory. In the end it will be more economical. A sharp, quite active campaign against the Indians during the coming fall and winter will be all that is needed, provided we have troops enough. The extent of country (Arizona) is so great and the number of Indians comparatively so small that they can evade the troops. Whilst we are scouting in one section they are depredating in another, but with troops enouch to operate in all sections at the same time a short campaign will suffice.

I cannot agree with the general in his estimate of the short duration of hostilities in his district, though I do in the economy, in every point of view, of his having as large a force as can be used and supplied. I therefore with two regiments of infantry for Arizona. I can spare from the troops at the Presidio some companies and a company from Southern California, and shall immediately order them to proceed to Arizona. Ultimaterly I will send four other companies from Southern California, making in all thirteen companies, all of which in a few months' time will not amount to more than a regiment. For the other regiment I beg to suggest as follows: There are in New Mexico parts of the First and Fifth California Volunteer Infantry and part of the First California Volunteer Cavalry, which it is desirable should be drawn into Arizona, where they will be nearer their homes by the time their terms of service expire. That this men be done, and at the same time the brigadier-general commanding in New Mexico may have sufficient force to co-operate efficiently with the commander in Arizona, two full regiments of infantry, or their equivalent and two squadrons of cavalry should be sent to New Mexico, as it may be too late by the time this communication reaches you and can be acted upon for these troops to go from Kansas, either by the Cimarronor Raton routes. It may be necessary to send them from or through Texas, if that State is in a condition to admit of it. I have as yet received no reports or returns from New Mexico, and cannot write with any precision as to the number, disposition, or kind of troops in that district, and the number I have named is therefore a matter of conjecture, but cannot, I think, be far out of the way. In connection with this subject, I beg to ask that authority be given to consolidate the regiments of volunteer infantry in California with each other as they fall below the minimum, instead of reducing the regiments into battalions, so that some of the colonels, who are very necessary for holding commands, may be retained.

I have the honor to remain, general, very respectfully, your most obedient servant,

IRVIN McDOWELL,

Major-General, Commanding Department.