purpose of ascertaining the dimensions of the top. To the lad's astonishment, however, there was no summit. That is to say, the altar was hollow.

The professor exhibited considerable scientific excitement on hearing this. The man of science had been greatly puzzled over the total absence of any traces of the human sacrifices he knew must have taken place there. He now hailed Jack eagerly.

"Are there not some bones or traces of sacrifices inside it, my boy?" he inquired excitedly.

"Nary a bone," shouted Walter cheerfully.

"Hold on, though," cried Jack. "There are some queer-looking things down in one corner."

Lowering himself inside the altar, he made for one corner of the erection, in which he had spied a heap of fragile-looking bones of some kind.

"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."