40 CFR §52.1382

1. (a)The Montana plan, as submitted, is approved as meeting the requirements of Part C, Subpart 1 of the Clean Air Act, except that it does not apply to sources proposing to construct on Indian Reservations.
2. (b)Regulation for preventing significant deterioration of air quality. The provisions of § 52.21 except paragraph (a)(1) are hereby incorporated and made a part of the Montana State implementation plan and are applicable to proposed major stationary sources or major modifications to be located on Indian Reservations.
3. (c)
   1. (1)Except as set forth in this paragraph, all areas of Montana are designated Class II.
   2. (2)The Northern Cheyene Indian Reservation is designated Class I.
   3. (3)The Flathead Indian Reservation is designated Class I.
   4. (4)The Fort Peck Indian Reservation is designated Class I.