Subjoined is the letter of Indian Agent TWISS detailing the circumstances attending the settlements of the Mormons on the Upper Platte:INDIAN AGENCY OF THE UPPER PLATTE, ON RAW-HYDE CREEK, Monday July 13, 1857.

SIR : In a communication addressed to the Indian office, dated April last, I called the attention of the Department to the settlements being made within the boundaries of this agency by the Mormon church, clearly in violation of law, although the pretext or pretence under which these settlements are made is under cover of a contract of the Mormon church to carry the mail from Independence, Missouri, to Great Salt Lake City.

On the 25th of May, a large Mormon colony took possession of the Valley of Deer Creek, 100 miles west of Fort Laramie, and drove away a band of Sioux Indians whom I had settled there in April, and had induced to plant corn.

I left that Indian band on the 23d of May, to attend to matters connected with the Cheyenne band in the lower part of the agency.

I have information from a reliable source that these Mormons are about 300 in number, have plowed and planted 200 acres of prairie, and are building houses for the accommodation of 500 persons, and have a large herd of cattle, horses, and mules.

I am persuaded that the Mormon church intend, by this plea, thus partially developed, to monopolize all of the trade with the Indians whilst within or passing through the Indian country.

I respectfully and earnestly call the attention of the department to this invasion, and enter my protest against this occupation of the Indian country, in force, and the forcible ejection of the Indians from the place where I had settled them.

I am powerless to control this matter, for the Mormons obey no laws enacted by Congress.

I would respectfully request that the President will be pleased to issue such order as in his wisdom and judgment may seem best, in order to correct the evil complained of.

Shortly after receiving the above letter, the acting Commissioner of Indian Affairs addressed a letter to the Secretary of the Interior, in which he cited the case of Mr. HALL, (a contractor to carry the mails from Independence to Santa Fe,) who was allowed merely to occupy his improvements on lands belonging to the Indians with the consent of the Indians, the department having no authority to grant him permission to extend his improvements, make use of the timber, or to cultivate land. In the Commissioner’s opinion, the act of March 3, 1855, granting to mail contractors in the Territories West of the Mississippi the privilege of occupying stations at the rate of not more than one for every twenty miles of the mail route, with a presumption covering 640 acres of land to be taken contiguously, had reference only to those lands to which the Indian title had been extinguished and surrendered to the United States, and not lands belonging to tribes, to which the titles had not been extinguished, and therefore was not applicable to the case of Mr. HALL, as his station was upon the lands of the Kansas Indians, to which their title had not been extinguished.

The Commissioner holds that the views of the office in regard to the case of Mr. HALL apply in the case of the Mormons, since the lands upon which they have settled are not the property of the government, but of the Sioux, to whom it was assigned by the treaty of Laramie on the 17th of September, 1851.