uired the professor.

Ralph extended the object for their inspection.

"A strange coin," cried Walt.

"Not so blamed strange, either," said Pete, picking it off the boy's
palm and examining it. "It's a Mexican peso."

"Then the men who were here were Mexicans?" cried Jack.

"Not so fast, my boy," admonished Pete. "Might as well say that every
feller who finds a Canadian dime in his pocket is a Kanuck. Say," he
suggested suddenly, "suppose you boys jes' see if you can find any
tracks around the base of the mesa."

They scattered and looked carefully about them, but the bunch-grass
grew in quite a broad belt all about, and no footmarks could be
discerned. Nor did a careful examination of the grass show any broken
or trampled blades, as would have been the case had ponies been there
recently.

"That decides it," announced Pete, after this last fact had been
ascertained, "whoever made those foot-marks wasn't here recent, that's
a fact. But who could they have been, and what brought them here?"

"Maybe Indians," suggested Ralph sagely.

"Yep, if Indians wore boots, which they don't," grinned Pete, while
poor Ralph colored to the roots of his hair over the general laugh that
arose at his expense.

"I think," announced the professor finally, "that it would be our best
plan to go ahead exploring the mesa. After all, there is nothing here
that can hurt us. Those ruffians of Black Ramon's have been driven out
of the country, and, anyway, they would not be likely to come here. As
for Indians, their reservation is many miles to the north-east.
Whoever was here, was either on a scientific quest, like ourselves, or
else unfortunately lost in the desert."

"Jes' ther same," grunted Pete, in a low voice that nobody overheard,
"I'd like ter know what all this means: Big, shadowy shapes flitting
around in ther night, and footsteps here in ther mornin'. It don't
look right."

He took a swift glance all about him. In every direction lay the
desert--glittering, far-reaching, lonely as the open sea. The only
break in the monotony came to the south--on the border--where stretched
the rocky, desolate ridge.

"No one wouldn't come here without an object," reasoned Pete to
himself, as they began the ascent of the narrow, tortuous trail, "now,
what in thunder could that objec' hev bin?"

CHAPTER V.

THE HOLLOW ALTAR.

"Magnificent indeed!"

The words, falling from the professor's lips, echoed hollowly again