"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 pa.s.sed tranquilly, and, with the exception of the duty of removing the carca.s.s 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 pa.s.sed 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 to 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 c.u.mbersome, moving object in their midst.

"There's a wagon!" he cried, "a big one, too, and surrounded by hors.e.m.e.n. 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 was going to be a long and difficult task to get the professor out of his present predicament, and there seemed need for haste.

"Here, put this under your head," said Jack, stripping off his jacket hastily, and throwing it within, "I'll tell Coyote Pete about your accident, and we can get remedies from the packs."

But when Jack turned, only Ralph and Walt stood beside him. The st.u.r.dy cow-puncher had vanished.

"He's gone to get the gla.s.ses," explained Walt.

Presently Coyote Pete, very much out of breath from his dash down the path and up again, stood beside them. He had the gla.s.ses in his hand, and lost no time in applying them to his eyes. He had not had them there two minutes when he gave a quick exclamation and turned hastily to the boys.

"Lie down; lie down, every one of you," he ordered sharply.

They lost no time in obeying, as they knew that the old plainsman must have an excellent reason for such a command. The next instant Pete himself followed their example. Crouching low, he once more peered through the gla.s.ses above the edge of the cup-like depression.

"Who are they?" asked Jack in a low voice, wriggling his way to Pete's side.

"I'm not sure yet, but they are all armed. I caught the flash of sunlight on their rifles. If they are Mexican insurrectos, we are in a bad fix."

"Mexicans! What would they be doing this side of the border?"

"That remains to be seen. But I don't like the looks of it."

"Suppose they are Mexicans, Pete, would they do us any harm?"

"That depends a whole lot on whether they are on lawful business or not."

"You mean----"

"That I don't like the looks of it. If there's an insurrection in Mexico, those fellows are after no good on this side of the border.

They may be some band of cut-throats, who are taking advantage of the disturbances to raise Cain."

"Good gracious," exclaimed Jack, "and the professor's just injured himself so that we can't move him for some time anyhow."

Coyote Pete turned sharply on the boy.

"What's he done?"

"Broken his ankle, or, at any rate, seriously sprained it."

Pete's rejoinder to this was a long whistle of dismay. He said nothing, however, but once more applied the gla.s.ses to his eyes. Jack saw him gnaw his moustache, as he gazed out over the desert. The dust-cloud was quite close now--not more than a mile away. The boys, with their naked eyes, could easily catch the moving glint of metal.

"Well, Pete, what do you think?" inquired Jack eagerly, as the cowpuncher at length set down the gla.s.ses.

"That we're in Dutch," was the expressive rejoinder.