...'s Pass on the Frederick road. Two days after Lee left Frederick, McClellan occupied it, and at eleven o'clock on the night of the 13th informed Halleck that an order of General Lee's, addressed to D. H. Hill , had accidentally fallen into his hands, the authenticity of which he thought was unquestionable. It discloses, said he, some of the plans of the enemy, and shows most conclusively that the main r...