CHAPTER I. A SHOT FROM AMBUSH. Behind a big rock which looked down over the wide, straggling road that ran upward through the mountains crouched a long, lean figure. Snuggled against his right shoulder was a rifle, and the bearded face beneath the broad-brimmed panama was turned toward the roadway below. The hot sun beat down remorselessly, and its blinding rays were reflected from the rocks. Perspiration poured down the man’s face, and now and then he moved impatiently to brush away some buzzing insect. His head was raised slightly above the level of the rock, and from his point of vantage a splendid panorama spread out beneath him. To his left lay the mountains, blue, remote, and full of rugged dignity all their own. To his right, a fertile South American valley revealed itself in the shimmering distance. Occasionally, as a light puff of wind came up from the lowlands, it brought with it the dull, heavy noise of an engine at work. Half an hour passed, and then the first sign of life was revealed in the roadway below. There appeared round a bend a long line of mules, each of them burdened with two big packs. In front of the train of mules walked a white man clad in dingy overalls. The figure crouching behind the rock moved slightly and seemed to grow tense and expectant, while the eyes in the bearded face glinted as they peered down at the road. Nothing happened, however. The mules plodded on, with their leader striding away ahead of them, and the lonely sentinel watched them until they had passed down the road and had vanished below the level of the rise which led them on to the plains. “He ought to be coming soon now.” The man spoke aloud, and there was a curious, metallic sound in his rasping voice. Ten minutes passed, and then the clear, drumming sound of a horse’s hoofs came to him, and presently around the same jagged spur there appeared the figure of a man on horseback. He was riding along at a good pace, but the reins were lying loosely across the horse’s neck, and the animal was picking its way unguided down the rough surface of the road. Evidently it was on a familiar trail. At the sight, the lurking figure grew tenser still, and the sound of a low growl, almost animal-like in character, might have been heard. Slowly the rifle was nestled closer to the shoulder. The panama hat, being too conspicuous, was pulled off and dropped behind him, after which the bare, rather bald head was lowered until the right cheek touched the stock of the gun. The left eye was closed, and the right sighted along the barrel, which at the same time was shifted, following the man on horseback. A few moments passed, during which the down-pointed muzzle shifted like a spy-glass, following the moving object. Then——