SALT LAKE, March 23, 1863.

Lieutenant Colonel R. C. DRUM:

Overland mail attacked by Humboldt Indians 100 miles east of Ruby. Driver and two station keepeers killed. Am taking measures to prevent recurrence.

P. EDW. CONNOR,

Colonel Third California Volunteer Infantry.

HEADQUARTERS DEPARTMENT OF THE PACIFIC,
San Francisco, March 24, 1863.

Brigadier General L. THOMAS,

Adjutant-General U. S. Army, Washington, D. C.:

GENERAL: For some weeks past suspicions have been entertained that attempts were being made to fit out rebel cruisers, either on Puget Sound or in the harbor of San Francisco, for the purpose of preying upon our commerce on the Pacifc. The collector of this port and officials of the Government, as well as the city authorities, were on the alert to delect any parties who might engage in such a busines. A schooner in the harbor attacked attention. She was loading for the Port of Manzanillo, Mexico, and finaly cleared at the custom-house with an assorted cargo of merchandise, including machinery, &c., for the above-named port, having an ordinary crew and no passengers. After getting under way the schooner was seized by an armed party from the U. s. ship Cayne at the request of Collector Rankin. The schooner was towed to Alcatraz Island. Besides the crew some fifteen men were found secreted on board. The bills of lading under which she had obtained her clearance were found to be false. Instead of machinery as invoiced, she had cannon and munitions of war on board. Everything was landed on the island, and all the men found on the vessel were placed in confinement separately. I then directed the commanding officer at the island to examine each person found on board, in order to ascertain as far as possible their object and designs, and particularly as to whether they had any confederates, ro had any appointed place of rendezvous for vessels of like character, either on the coast or adjacent islands. At the movement of seizure there was a man who was in charge of the schooner, who was to have been, after going to sea, the third officer. This man has turned State's evidence, and his statement is in substance that some months since he was approached by a man by the name of Harpending-one of the leaders and now in confinement-who exhibited to him a letter of marque signed by Jefferson Davis and a Secretary whose name he does not recollet, together with a letter of instructions as to the disposal of captured prizes, &c,: that the at first declined but subsequently agreed to join the expedition; that at the movement of seizure of the vessel he saw this man Harpending chewing and tearing up papers, which he doubts no were the letter of marque and instructions referred to.

The statements of Harpending and his leading confederates, so far as they will answer at all, are that they were engaged in a legitimate traffic, on a speculation to the coast of Mexico. The statements of the men amount to nearly the same thing, that they were engaged to go to Mexico for the purpose of mining, &c. ; most of them however doubtless knew full well the business upon which they were to be employed. I have these men all confined on Alcatraz Island and after completing the investig nations it will then be considered before what tribunal it may be proper to arrign them. The discovery of this plot to prey upon