Mont. Code Ann. § 2-1-203

2-1-203 . Withdrawal of offer to cede legislative jurisdiction. The offer by the state of Montana to cede to the federal government legislative jurisdiction over areas within the state of Montana as contained in the act of the second legislative assembly of the state of Montana, 1891, entitled: "An Act Giving the Consent of the State of Montana to the Purchase, by the United States, of Land in any City or Town of the State, for the Purpose of United States Courthouse, Post Office and for Other Purposes" approved March 5, 1891, as amended by the act of the third legislative assembly of 1893, an act entitled: "An Act Giving the Consent of the State of Montana to the Purchase by the United States of Land in any City or Town of the State for the Purpose of United States Courthouse, Post Offices and for Other Like Purposes", approved March 9, 1893, is hereby withdrawn except as to areas heretofore completely purchased or acquired by the federal government and over which areas the federal government has heretofore assumed either exclusive legislative jurisdiction or concurrent legislative jurisdiction under the terms of one or the other of said acts.