"Skeletons of snakes!" he cried, holding up one of these for the inspection of the professor, who had by this time hoisted his bony frame over the top of the altar and now stood beside them. "That's right, my boy; they are serpents' skeletons. Doubtless in their sacrificial ceremonies these people also offered up rattlesnakes, which seem to have been a sort of sacred reptile among them; much as, in a sense, the cat was sacred to the ancient Egyptians, and the python is worshiped in certain parts of India." "But, professor," protested Jack, "if, as you say, numerous human sacrifices were offered here in the past, why do we not find any human remains here?" "Who can say, my boy? Many of the habits of these pre-historic peoples are veiled in mystery. We can only surmise and reconstruct. They may have burned them or disposed of them in some other way." "Say!" exclaimed Ralph suddenly. "This floor sounds to me as if it was hollow; maybe there's a chamber or something underneath." The boy, who had been stamping about with a vague sense of making some such discovery, hailed them with excited looks. "Hollow, you say?" asked the professor, with every appearance of deep interest. "Yes, listen!" Again Ralph stamped about. There was no question about it--the stone-paving, of which the floor of the altar was formed, gave out an unmistakably hollow sound. The professor was down on his hands and knees instantly, searching about, like a hound on the scent. In the meantime the others stamped about in other parts of the interior, but only where Ralph's feet had given out the hollow sound did the floor appear anything but solid. "Queer!" exclaimed the professor, as, after a considerable search, he rose to his feet covered with dust and streaming with perspiration, "there should be some sort of trap-door here, to judge by the sounds, but so far as I can see, the joints between the pavement are perfectly tight, and I can find no ring or lever which might open such an aperture." "Perhaps----" began Ralph, but he was interrupted by a sudden wild yell from Pete. "Wow! Yee-ow! Come here quick, everybody!" CHAPTER VI. THE LEGEND OF A FORGOTTEN RACE. Leaping and scrambling over the top of the hollow altar to the best of their abilities, the four explorers found their cow-puncher friend dancing wildly about on the edge of the mesa, in imminent peril of tumbling over altogether. He was wildly excited, and, as they emerged, he pointed down over the cliff edge. "Whatever is the matter?" exclaimed Jack, regarding the antics of the usually staid cow-puncher with amazement. "The stock! Look at the stock!" yelled Pete.