Cite as: 522 U. S. 329 (1998)
Opinion of the Court
country.' " Rosebud, supra, at 613. By 1892, Congress already had enacted laws prohibiting alcohol on Indian reservations, see Cohen 306-307, and "[w]e assume that Congress is aware of existing law when it passes legislation," Miles v. Apex Marine Corp., 498 U. S. 19, 32 (1990). Furthermore, the Commissioner of Indian Affairs described the provision as prohibiting "the sale or disposition of intoxicants upon any of the lands now within the Yankton Reservation," Letter, at 6-7 (emphasis added), indicating that the lands would be severed from the reservation upon ratification of the agreement. In Perrin v. United States, 232 U. S. 478 (1914), we implied that the lands conveyed by the 1894 Act lost their reservation status when we construed Article XVII as applying to "ceded lands formerly included in the Yankton Sioux Indian Reservation." Id., at 480. We now reaffirm that the terms of the 1894 Act, including both the explicit language of cession and the surrounding provisions, attest to Congress' intent to diminish the Yankton Reservation.
III
Although we perceive congressional intent to diminish the reservation in the plain statutory language, we also take note of the contemporary historical context, subsequent congressional and administrative references to the reservation, and demographic trends. Even in the absence of a clear expression of congressional purpose in the text of a surplus land Act, unequivocal evidence derived from the surrounding circumstances may support the conclusion that a reservation has been diminished. See Solem, 465 U. S., at 471. In this case, although the context of the Act is not so compelling that, standing alone, it would indicate diminishment, neither does it rebut the "almost insurmountable presumption" that arises from the statute's plain terms. Id., at 470.
A
The "manner in which the transaction was negotiated" with the Yankton Tribe and "the tenor of legislative Reports
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