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-grass 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 m