Both drew rein for a second and listened. The beat of hoofs in front of them suddenly slackened. So near was the sound that it seemed as if it could not have been more than a few feet ahead. "Right through that brush there!" whispered Clark, and hot as the day was, he shivered as if stricken with a sudden fever. Bill Bender coolly raised his rifle. He deliberately aimed it into the leafy screen. The next instant its deafening report rang out. It was followed by a loud crash from beyond the bushes, as if some heavy body had fallen. Clark fairly turned his pony round. He was too much of a coward even to dare to ask the question that forced itself to his lips. No such qualms assailed Bill Bender, however. He pressed spurs to his pony, and in a second flashed round the trees that hid what lay on the trail beyond. A second later a loud cry of astonishment broke from his lips. It was mingled with curses. "What's the matter?" hailed Clark tremblingly. "Come here." "Oh, Bill, I don't want to. I----" "Come here, I say. There's nothing to be afraid of." Thus urged, Clark, whose cheeks were still ashen under the bronze, urged his pony forward, and presently joined Bill. The latter had dismounted, and was standing over a dark, still object in the road. It was the pony Rob had borrowed so hurriedly. It lay stone dead, pierced in a vital spot by Bill Bender's bullet. "But the b-b-boy, is he----" stuttered Clark. "He's gone!" exclaimed Bill. "Gone?" echoed Clark in an amazed tone. "Yes, clean wiped out." "But how?" "Ask me an easy one." "Hasn't he left a trail?" "No, that's what makes it so queer. He must have had an aeroplane." For half an hour or more both youths searched the dusty trail and beat in and out of the dense brush, but not a trace of the missing boy rewarded their close scrutiny of the surroundings. Had the earth opened at that spot and swallowed Rob up bodily, he could not have vanished more utterly. The only trace of the missing boy was his sombrero, lying by the dead pony. Absolutely dumfounded with amazement, the two worthies finally gave up their search, and taking the saddle and bridle off the dead pony, made their way back to their camp, carrying Rob's broad-brimmed hat. * * * * * At about the same hour that Clark and Bill were searching among the piñon and scrub growth for some solution of the mystery of Rob's inexplicable disappearance, an equally perplexed party was assembled on a small rise some miles away. The latter group consisted of Mr. Harkness, his son, the Boy Scouts of the Ranger Patrol, Corporal Merritt Crawford and Tubby Hopkins, Blinky and two other cow-punchers.