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.

But all this time we have been allowing our party to travel on without bestowing any attention upon them. As the afternoon wore on, Coyote Pete began to feel real apprehension about reaching their destination that evening. Walt Phelps' fear about the water had been verified. The supply was getting low. Provided they could "pick up" the mesa they were in search of before sundown, however, this was not so serious a matter as might have been supposed. Coyote Peter knew that there was a well at the mesa, the handiwork of the ancient desert-dwellers.

The really serious thing was, that although they had apparently been traveling in the right direction, they had not yet sighted it. The cow-puncher knew, though he did not tell his young companions so, that they should long since have spied its outlines. Of the real seriousness which their position might shortly assume, the boys had as yet, little idea. Coyote Pete was not the one to alarm them unless he was convinced it was really necessary.

Suddenly, Jack, who had been riding a little in advance of the rest, gave an exclamation and pointed upward at the sun.

"Say, what's the matter with the sun?" he exclaimed.

"Sun spots, I suppose," put in Ralph Stetson jokingly.

"I see what you mean," spoke up the professor; "it has turned quite red, and there seems to be a haze overcasting the sky."

"It's getting oppressive, too," put in Walt Phelps. "What's up, Pete?"

The cow-puncher had, indeed, for some time been noticing the same phenomenon which had just attracted their notice, but he had hesitated to draw their attention to it. Now, however, he spoke, and his voice sounded grave for one of Pete's usually lively temperament.

"It means that ole Mar'm Desert is gettin' inter a tantrum," he grunted, "and that we're in an almighty fix," he added to himself.

"Is it going to rain?" inquired Ralph Stetson, as it grew rapidly darker.

"Rain?" grunted Pete. "Son, it don't rain here enough to cover the back uv a dime, even if you collect all the water that fell in a year. No, siree, what's comin' is a heap worse than rain."

"An electric storm?" queried the professor.

"No, sir—a sand storm," rejoined the cow-puncher bluntly.

CHAPTER II.

THE SAND STORM.

As he spoke, a queer, moaning sort of sound, something like the low, distant bellow of a steer in pain, could be heard. The air seemed filled with it. Coming from no definite direction, it yet impregnated the atmosphere. The air, too, began noticeably to thicken, until the sun, from a pallid disc—a mere ghost of its former blazing self—was blotted out altogether. A hot wind sprang up and swept witheringly about the travelers.

"Ouch!" exclaimed Ralph Stetson suddenly. "Something stung me!"

"That's the sand, son," said Coyote Pete. "The wind's commencin' ter drive it."

"Is it going to get any worse?" inquired the professor anxiously.

"A whole lot, afore it gits any better," was the disconcerting reply.

"What'll we do, Pete?" asked Jack, turning to the cow-puncher.

It had now grown so dark that he could hardly see Pete's face. It was hot, too, with a heavy, suffocating sort of heat. The wind that drove the myriads upon myriads of tiny sand grains now darkening the air, was ardent as the blast from an opened oven-door.

"Get your saddles off, quick! Lie down, and put your heads under 'em," ordered the cow-puncher, briskly swinging himself out of his saddle as he spoke.

The others hastened to follow his example. It was not a minute too soon. Already their mouths were full of gritty particles, and their eyes smarted as if they had been seared with hot irons. The ponies could hardly be induced to stand up while the process of unsaddling was gone through. As for the burros, those intelligent beasts had thrown themselves down as soon as the halt was made. With their heads laid as low as possible, and their hind quarters turned to the direction of the hot blast, they were as well prepared to weather the sand storm as they could be.

The instant the saddles were off the ponies, down they flopped, too, in the same positions as their long-eared cousins. The bipeds of the party made haste to follow their animals' example, only, in their case, their heads were sheltered as snugly as if under a tent, by the big, high-peaked, broad-flapped Mexican saddles.

It was well they had made haste, for, as Pete had said, the sand storm was evidently going to get "a whole lot worse before it got better." The air grew almost as black as night, and the wind fairly screamed as it swept over them. Jack could feel little piles of sand drifting up about them, just as driven snow forms in drifts when it strikes an obstruction. How hot it was under the saddles! The boys' mouths felt as if they would crack, so dry and feverish had they become.

"Oh, for a drink of water!" thought Jack, trying in vain to moisten his mouth by moving his tongue about within it.

All at once, above the screaming of the wind, the lad caught another sound—the galloping of hoofs coming toward them at a rapid rate. For an instant the thought flashed across him that it was their own stock that had stampeded. He stuck his head out to see, braving the furious sweep of the stinging sand.

He withdrew it like a tortoise beneath its cover, with a cry that was only half of pain. Through the driving sand he had distinctly seen three enormous forms sweep by, seen like dim shadows in the gloom around. What could they have been? In vain Jack cudgeled his brains for a solution to the mystery.

The forms he had seen drift by had been larger than any horse. So vague had