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."

"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.

Peering over the edge at the bunch-gra.s.s belt in which their ponies were tethered, the adventurers saw a spectacle which might well have been calculated to excite the cow-puncher. One Spot, Two Spot and Three Spot were tearing round and round at the end of their tethers, in the wildest alarm, evidently, while the cayuses were stamping and snorting, with distended nostrils and wild, frightened eyes.

"What's the matter with them?" gasped Walt, astonished at the sight, as well he might be. The desert was as empty as ever, and there was no sign of anything in the rocky hills to the south that might have excited their alarms.

"Thet's jes' it," said Pete. "What is the matter with 'em? They ain't actin' up thet er way fer nuthin', you kin bet."

"Something must have scared them," said Jack. "Maybe it was those rocks you were throwing down."

"No, it warn't that, son. Ole One Spot he looked up here a minute ago, and giv' his eye a knowin' wink, as much as ter say: 'Go ahead; I know you won't hurt us.' No, siree; it's suthin' they've smelled out, er seen, that's given 'em the scare of their young lives."

"Maybe it was something on the other side of the mesa. Let's go and look," cried Jack.

Followed by the others, he ran across the flat summit, but an earnest inspection of the surroundings on that side failed to reveal any explanation for the animals' sudden terror. For all the strange objects that lay about them, they might have been in the middle of a desolate ocean.

"No wonder they call this the Haunted Mesa," snorted Pete. "I tell you, perfusser, ther sooner you git them thar measurements a-measured, and we're hiking out of this neck of the woods, the better I'll be pleased. 'Tain't natural, all these queer goings on."

"Maybe a coyote or something scared them," suggested Ralph.

"And them used ter seeing 'em every day," scoffed Pete. "Guess again, son. It takes something with hoofs, horns and red fire about it to scare a burro, and you kin bet your Sunday sombrero on that."

"Well, I propose that we adjourn the meeting till after dinner,"

laughed Jack; "all in favor, will signify by saying 'aye.'"

The chorus that answered him left no doubt of "the sense of the meeting," and a rapid descent of the mysterious mesa was begun. A good meal was not long in being prepared, thanks to Coyote Pete's skill as a camp cook. Seated over their dinner, the main topic of conversation was naturally the unaccountable occurrence of the morning. But although a score of explanations were advanced, n.o.body could hit on one that seemed to fit the case.

"This water is singularly pure and sparkling,"' said the professor finally, by way of changing the subject, and holding up his full tin cup.

"Yep; I remember hearing old cowmen say that there's no water in New Mexico any better than this from the Haunted Mesa," said Pete, stretching himself out, and lighting his inevitable after-meal-time pipe. "Though that ain't sayin' a heap," he admitted.

"Wonder how those old what-you-may-call-ums ever managed to dig such a well?" questioned Ralph.

"Comes to my mind now," said Pete, "that it ain't exactly a well. An old Injun that used ter hang around with the Flying Z outfit tole us oncet that thar was a subterranean river flowed under here, and that once upon a time afore all the country dried up, considerable more water came to the surface here than there does now."

"A subterranean river?" asked the professor, at once interested.

"Yes, sir," rejoined Pete, "and not the only one in the West, either.

There's one in Californy that flows underground fer purty near fifty miles, as I've heard tell."

"This is most remarkable," said the professor. "I, too, have heard of subterranean rivers in this part of the world, but I have never had the opportunity to explore one. Did this Indian you speak of ever tell you where this river emerges?"

"He said it come out some place across the frontier in Chihuahua; I don't jest rightly recollect where," said Pete carelessly, as if the subject did not interest him much, as indeed it did not.

"I don't see what use a subterranean river is to anybody, anyhow," he went on. "If it was on top, now, it might be some use."

"But this is most interesting," protested the professor, while the boys lay about with their chins propped in their hands in intent att.i.tudes.

"Then, too, if this river exists, perhaps it is even navigable."

"Why, professor!" exclaimed Jack. "Is it not possible that it was to this river that those drawings of boats that interested and puzzled you so much had reference?"

"Quite possible, my boy," agreed the man of science.

"I wish we could find some way of getting down into it," said Ralph wistfully, poking at the ground, as if he thought he might force an entrance that way.