o the
boys.

"What's coming--another storm?" asked Ralph.

"I don't know what it is yet," rejoined the other in a strangely uneasy
tone, "it looks like--like----"

"A pillar of dust," exclaimed Jack, who had by this time sighted it,
too, and had come to the aid of the unimaginative plainsman.

"So it does," cried the others, who now, with the exception of the
short-sighted professor, could also see the approaching dust-cloud.

"What can it be?" wondered Walt, peering eagerly in its direction.

"Somebody riding. Several of 'em, I should say, by the dust they're
raising," rejoined Pete bluntly.

The boys exchanged quick glances. Somebody riding across that arid
waste? Their destination could only be the mesa, then, but who could
it possibly be?

Had they been able to solve the riddle at that instant, they would have
scattered pell-mell for their ponies, and made the best of their way
from the Haunted Mesa, but, not being endowed with anything more than
ordinary sensibilities, it was, of course, impossible for them to
realize the deadly peril that was bearing down upon them in that
dust-cloud.

"I can see things more clearly now," cried Jack, as for an instant a
vagrant desert air blew aside the dust-cloud and revealed several
riders, surrounding some cumbersome, moving object in their midst.

"There's a wagon!" he cried, "a big one, too, and surrounded by
horsemen. What can it mean?"

"That we'd better be skedaddling as quick as possible," shot out Pete,
brusquely.

The professor, who had wandered away from the group and was down inside
the hollow altar, was hastily summoned and apprised of the strange
approach of the mysterious cavalcade.

"Why, bless me, boys, what can it mean?" he cried, nimbly attempting a
flying leap over the edge of the altar in his haste to ascertain for
himself the nature of the approaching party.

Suddenly, however, as his feet touched the top, and he was scrambling
over, he gave a sharp cry and fell back within the altar with a gasp of
pain.

"Are you hurt?" asked Jack, running to the side of the ancient place of
sacrifice.

The professor lay prostrate within. His face was white and set and
beads of sweat stood out on his forehead.

"My--my ankle," he groaned. "I broke it some time ago, and in hurrying
to clamber over the top of the altar I fear I have snapped it again.
Oh!"

He gave a heartrending groan of pain. The boys stood stricken with
consternation. It wa