"We're not likely to find a better place," announced Jack, after a hasty inspection. "Turn the horses loose," he cried in a low, but penetrating voice, down to Walt, who had remained below with the stock. The red-headed ranch boy slipped off the back of his steed and alighted on a rock, so as to make no tracks. He then gave the three horses, that had borne them so bravely, their liberty. At first the animals would not move, but began cropping the green stuff about them. "Here, that won't do," breathed Jack, as the three lads crouched at the cave mouth. "Throw some rocks at them, Walt." The boys picked up some small stones, which lay littered in front of the cave, and commenced a fusillade. It had such good results, that a few seconds later, the three horses were plunging off along the bottom of the gully as if Old Nick himself had been after them. As their hoof-beats grew faint, Jack held up his hand to enjoin silence, although the boys had been discussing their situation in such low tones that their voices could not have traveled ten feet from the cave mouth. "Hark!" he said. From farther down the gully came shouts and yells, and then the distinct rattling sound of loose shale, as several horsemen descended the steep bank into the gulch. "They've picked up the trail," commented Walt grimly. CHAPTER XVI. WHAT HAPPENED TO COYOTE PETE. Let us now retrace our steps to the Haunted Mesa, and ascertain how it fared with Coyote Pete and the professor, after the boys' astonishing disappearance through the balanced trap-door in the base of the hollow altar. As we know, the lads' elders were crouched at the opposite end of the former sacrificial structure, when, before their eyes, the lads were swallowed up. For an instant--as well they might have been--the two onlookers were fairly paralyzed with amazement. The occurrence seemed to be without natural explanation. But an investigation by Pete, crawling on his hands and knees while he made it, soon revealed the nature of the device which, as we know, was nothing more nor less than a balanced trap-door of stone. An unusual weight placed upon one end of it instantly tilted it and projected whatever was on it upon the staircase below. The professor, who recalled having read of such devices in other dwelling-places of ancient communities, was at first for following the boys into the unknown interior of the mesa, but before any move could be made in that direction, one of the newly-arrived party shoved his face over the top of the hollow altar in a spirit of investigation. He fell back with a yell, crying out that there were spirits within it, as his eyes encountered the crouching forms of its two occupants. "What's the matter, you fool?" demanded Ramon himself, who happened to be close at hand. "Oh, the spirits! The spirits of the hollow altar!" howled the Mexican in abject terror, his knees knocking together and his face taking on a sickly pallor.