Within the following-described areas any lands not in Government ownership which are found by the Secretary of Agriculture to be chiefly valuable for national-forest purposes may be offered in exchange under the provisions of sections 485 and 486 of this title, upon notice as therein provided and upon acceptance of title, shall become parts of the Plumas National Forest; and any of such described areas in Government ownership found by the Secretaries of Agriculture and the Interior to be chiefly valuable for national-forest purposes and not now parts of any national forest may be added to said national forest as herein provided by proclamation of the President, subject to all valid claims and provisions of existing withdrawals: Provided, That any lands received in exchange under the provisions of this section shall be open to mineral locations, mineral development, and patent in accordance with the mining laws of the United States:

Township 18 north, range 7 east, Mount Diablo base and meridian, California: Sections 3, 4, 5, 9, 10, 11, 12, 13, 15, 22, 23, 26, and 27.