"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 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 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.