With Black Ramon, thanks to the boys, forever banished from his cattle-rustling raids, and the subsequent tranquility of routine life, had come a recollection of the professor's quest. Coyote Pete, a few days before this story opens, had volunteered to act as guide to the professor and his party to a mesa seldom visited except by wandering Indians and occasional cow-punchers. This was the Haunted Mesa, the location of which was so difficult to reach that previous relic-hunting expeditions had not included it in their travels. Mr. Merrill was the more willing to allow the boys to go along, as he had been suddenly summoned into Chihuahua province, in Mexico, by reports of trouble at a mine--The Esmeralda--he owned there. Rumors of an insurrection had reached him--an insurrection which meant great peril to American interests. He had, therefore, lost no time in setting out to ascertain the true state of affairs at his mine, which, while a small one, was still likely to develop in time into an extremely valuable property. Leaving the ranch in charge of Bud Wilson, he had started for the Mexican country without waiting for the departure of the professor's expedition. A short time later, "Professor Wintergreen's Haunted Mesans," as the boys insisted on calling themselves, had likewise started on their quest. With them, at Jack Merrill's invitation, went Walter Phelps, the son of a ranching neighbor of Mr. Merrill. Walt, it will be recalled, had shared the perils and adventures of the boys across the border, as related in the previous volume, and had been the instrument of piloting them out of the mysterious valley in which Black Ramon kept his plundered herds. Mr. Merrill's last words had been ones of caution. "Remember, boys, that if this trouble in Mexico attains real proportions, life and property along the border may be in great danger. In such a case, it will be your immediate duty to turn back." "But, Dad," Jack had said, "you don't expect that plundering insurrectos would have the audacity to come northward into the Playas?" Mr. Merrill laughed. "I didn't say there was any danger even here, my boy. Least of all, out in that barren country. If there is an insurrection, it will doubtless be put down without any trouble, but it is always well to be prepared." Like his brother ranchers along the border, Mr. Merrill at that time had no idea of the seriousness or extent of the insurrection. Had he had, he would, of course, have prohibited the party leaving the ranch. As it was, he, in common with his neighbors, deemed the insurrection simply one of those little outbreaks that occur every now and again in Mexico, and which hitherto had been promptly squashed by Diaz's army. And so, with no real misgivings, the party had bidden the bluff, good-natured rancher good-by, little dreaming under what circumstances they were to meet again.