The State of Arkansas consents to the purchase by the United States of a site for a public building mentioned in 26 Stat. 702, Ch. 39, and cedes to the United States exclusive jurisdiction over any lot or block of ground within the corporate limits of the City of Camden, not exceeding in area three hundred sixty feet (360') square, which may be so purchased by the United States, during the time the United States remains the owner thereof, for all purposes except the administration of the criminal laws of the state and the service of civil process therein.