them?" asked Ralph, who had been
apprised by Jack of the strange vanishment of the dead creature's mates.

"Must uv gone down that draw I noticed frum ther top uv ther mesa
to-day," explained Pete. "Yer see, frum here, it would look as if they
vanished inter the solid earth when they entered it, bein' as how you
can't see there's any kind of a gully there till you get up high."

The next morning this was found to be the true explanation. Tracks on
the bottom of the gully showed plainly how the strange desert wanderers
had effected their disappearance in such a startling manner. But it
was some time before Pete could sit down to a meal without being
reminded of his "fire-spouting spook," which had cast such alarm into
the camp the first night. The boys spent a week more at the mesa,
during which time Professor Wintergreen obtained voluminous notes on
one of the most interesting specimens of its kind in the south-west.

The days passed tranquilly, and, with the exception of the duty of
removing the carcass of the dead camel, nothing to interrupt the
routine of survey work occurred. The mates of the dead beast had
evidently decided not to revisit their pasture grounds, for they did
not put in a reappearance.

"Well, boys," said the professor one morning when they were all
gathered at the summit of the mesa, "I guess that to-morrow morning we
can say good-by to the scene of our rather tame adventures. My work is
complete."

"How about the subterranean river?" asked Ralph, but a howl of derision
from the others silenced him.

"Subterranean fiddlestick," burst out Jack, but the professor silenced
him.

"The existence of such a stream is not so improbable as you seem to
think," he said, "and Master Ralph is to be commended for his
enterprising desire to locate it, but I think that our investigations
have shown that if such a river ever did exist and the mesa dwellers
had access to it, that the entrance, wherever it might have been, has
vanished long ages ago."

Pete had taken no part in this conversation, but had wandered about the
top of the mesa rather aimlessly, from time to time looking sharply at
the surroundings beneath him in the alert manner of one whose life has
been passed in the open places.

Suddenly he gave a quick exclamation and pointed off into the
north-west.

"Look! Look there!" he exclaimed, riveting his eyes on something his
keen vision had sighted, but which remained as yet invisible t