Mont. Code § 2-1-202
Pursuant to Article I, section 8, paragraph 17, of the constitution of the United States, consent to purchase is hereby given and exclusive jurisdiction is ceded to the United States over and with respect to any lands within the limits of this state that are acquired by the complete purchase by the United States for any of the purposes described in paragraph 17 of the constitution of the United States. The jurisdiction must continue as long as the lands are held and occupied by the United States for the described purposes. The state reserves the right to serve and execute civil or criminal process lawfully issued by the courts of the state within the limits of the territory over which jurisdiction is ceded in any suits or transactions for or on account of any rights obtained, obligations incurred, or crimes committed in this state, within or outside of the territory. The state also reserves the right to tax persons and corporations and their franchises and property within the territory. The state and its inhabitants and citizens reserve the right to fish and hunt and the right of access, ingress, and egress to and through the ceded territory to all persons owning or controlling livestock for the purpose of watering the livestock. The state reserves jurisdiction in the enforcement of state laws relating to the duties of the department of livestock and the department of environmental quality and the enforcement of any regulations promulgated by the departments in accordance with the laws of the state. Jurisdiction does not vest until the United States, through the proper officers, files an accurate map or plat and description by metes and bounds of the lands in the office of the county clerk and recorder of the county in which the lands are situated. If the lands are within the corporate limits of any city, the map or plat must also be filed in the office of the city clerk of the city. The filing of the map constitutes acceptance of the jurisdiction by the United States as ceded.
§ 2-1-202, MCA