Derrick now came back, and with him Paul went to the door that he was to tend. Just inside of it, on a platform laid above the ditch of black, rapidly flowing water, stood a rude arm-chair made out of rough boards. Above it hung a board full of holes into which several pegs were thrust. Derrick told Paul that with these pegs he must keep tally of the number of loaded cars that passed this station, and that he must always be ready to answer promptly the call of "Door." Within reach from the chair was a lever by means of which the switch was moved. Paul was told that after each door call there would come another explaining on which track the approaching cars were to go, and that he must listen carefully for it and set the switch accordingly. After showing him the large oil-can from which he might refill his lamp, Derrick bade him good-by and returned to his own work. This morning passed much more pleasantly to the young mule-driver than the first one had. Not only did Tom Evert greet him cordially, and thank him for what he had done for Paul, but Monk Tooley gave him a gruff "Mornin', lad," and most of the other men spoke pleasantly to him, as though to atone in a measure for his previous suffering. Above all, he occasionally had to pass Paul's station, and the mere sight of his faithful friend leaning on his crutch and holding open the door was a source of joy. As Paul had much spare time on his hands, he occupied it in becoming acquainted with his surroundings, and was especially interested in the curious markings on the black slate walls of the gangway near his door. Many of these were in the form of exquisite ferns, others of curious leaves such as he had never seen, quaint patterns like the scales and bones of queer fishes, or the ripplings of water on a smooth beach. In one place he found tiny tracks, as though a small bird had run quickly across it, and had stamped the imprint of its feet on the hard surface. It was Paul's first lesson in geology, and it gave him his first idea that this hard slate, and the veins of coal enclosed between its solid walls, might have had a previous existence in another form. He pondered upon the length of time that must have passed since those ferns grew, and since that running bird made those footprints, and finally concluded to ask Derrick if he knew. At noon, after Harry Mule had been sent jingling to his stable, Derrick rejoined his friend, and they ate lunch together. As they talked of the strange markings on the walls, and Derrick confessed that he knew no more concerning their age than Paul, the latter suddenly paused, and with a slight gesture directed attention to something in the roadway. Looking in the direction indicated, Derrick saw, sitting bolt-upright on its hind-legs, and gazing steadily at them, an immense rat. He was quite gray, and evidently very old; nor did he seem to be in the least bit afraid of them. "Doesn't he look wise?" whispered Paul.