Upon receipt of a report from the planning commission, or upon the expiration of thirty (30) days after the conclusion of the hearing by the planning commission on any amendment initiated by petition or by resolution of the board of supervisors, the board of supervisors shall set the matter for public hearing, after notice thereof, given as provided by law. After the conclusion of such hearing, the board of supervisors may adopt the amendment of any part thereof set forth in the petition or in the resolution of intention in such form as the board may deem to be advisable. (Prior gen. code § 8-103.7)