PORTLAND, Ore., Jan. 23- Willamette Industries says that it has reached an agreement in principal for the sale of Willamette to Weyerhaeuser for $55.50 per share in cash, subject to negotiation of a definitive merger agreement. The Company also announced that negotiations with Georgia-Pacific Corp. have ended. The Weyerhaeuser agreement will be considered at a Willamette Board meeting expected to be convened prior to the end of January. The transaction is subject to the negotiation of a definitive merger agreement and approval by the Weyerhaeuser and Willamette boards of directors. In order to ensure that Willamette shareholders receive their money promptly, assuming a definitive agreement is reached, Weyerhaeuser expects to accelerate the closing of its outstanding tender offer.