Of one thing he was sure--that he could not have found his back-trail. But he divined he was never to retrace his steps on this journey. The stretch of broken plateau before him grew wilder and bolder of outline, darker in color, weirder in aspect, and progress across it grew slower, more dangerous. There were many places Nagger should not have been put to--where a slip meant a broken leg. But Slone could not turn back. And something besides an indomitable spirit kept him going. Again the sound resembling thunder assailed his ears, louder this time. The plateau appeared to be ending in a series of great capes or promontories. Slone feared he would soon come out upon a promontory from which he might see the impossibility of further travel. He felt relieved down in the gullies, where he could not see far. He climbed out of one, presently, from which there extended a narrow ledge with a slant too perilous for any horse. He stepped out upon that with far less confidence than Nagger. To the right was a bulge of low wall, and a few feet to the left a dark precipice. The trail here was faintly outlined, and it was six inches wide and slanting as well. It seemed endless to Slone, that ledge. He looked only down at his feet and listened to Nagger's steps. The big horse trod carefully, but naturally, and he did not slip. That ledge extended in a long curve, turning slowly away from the precipice, and ascending a little at the further end. Slone, drew a deep breath of relief when he led Nagger up on level rock. Suddenly a strange yet familiar sound halted Slone, as if he had been struck. The wild, shrill, high-pitched, piercing whistle of a stallion! Nagger neighed a blast in reply and pounded the rock with his iron-shod hoofs. With a thrill Slone looked ahead. There, some few hundred yards distant, on a promontory, stood a red horse. "My Lord! ... It's Wildfire!" breathed Slone, tensely. He could not believe his sight. He imagined he was dreaming. But as Nagger stamped and snorted defiance Slone looked with fixed and keen gaze, and knew that beautiful picture was no lie. Wildfire was as red as fire. His long mane, wild in the wind, was like a whipping, black-streaked flame. Silhouetted there against that canyon background he seemed gigantic, a demon horse, ready to plunge into fiery depths. He was looking back over his shoulder, his head very high, and every line of him was instinct with wildness. Again he sent out that shrill, air-splitting whistle. Slone understood it to be a clarion call to Nagger. If Nagger had been alone Wildfire would have killed him. The red stallion was a killer of horses. All over the Utah ranges he had left the trail of a murderer. Nagger understood this, too, for he whistled back in rage and terror. It took an iron arm to hold him. Then Wildfire plunged, apparently down, and vanished from Slone's sight.