Even the stolid face of the guide relaxed in a broad smile of amusement.

"Then, if you are all supplied, we had better be on our way. If we are going to camp on the alkali to-night we shall have to make time between this and sundown. It's about three hours high."

With a whoop and a hurrah, the boys swung into their saddles, heading joyously for the Newark Valley and the silent, mysterious desert that in the dim, misty past had been a great inland sea.

Readers of the preceding volumes of this series will recall how the Pony Rider Boys came to spend their summer vacation on horseback, under the guardianship of Walter Perkins' tutor, Professor Zepplin. With a capable mountain guide, their first journey was through the wildest part of the Rocky Mountains, where they met with a series of rousing adventures and hair-breadth escapes—experiences calculated to try the stoutest hearts. It was here that the young explorers hunted big game—here that they discovered a valuable mine that had been the goal of prospectors for many years past.

All this was outlined in the first volume of the series, "THE PONY RIDER BOYS IN THE ROCKIES." In the second volume, "THE PONY RIDER BOYS IN TEXAS," was narrated how the four lads joined in a cattle drive across the plains of Texas, becoming real cowboys. Being by this time well hardened physically, they were able to do men's work in rounding up the stampeding cattle, which led them into many thrilling adventures. It will be recalled, too, how, during a visit to the mysterious church of San Miguel, the Pony Rider Boys solved the veiled riddle of the plains, which marked the end of the most eventful journey of their lives. In the third volume, "THE PONY RIDER BOYS IN MONTANA," we find the plucky lads following the old Custer trail over mountain and plain. It will be remembered how Tad Butler, while chasing a bear that had disturbed their camp, overheard a plot to stampede and slaughter the herd of sheep belonging to a rancher whom they knew; how the lad managed to escape from the men who sought his life; his eventual capture by the Blackfeet Indians, his escape, and the final solving of the mystery of the old Custer trail, during which the boys were in the thick of a battle between cowboys and sheep herders. In the volume preceding the present one the Pony Rider Boys were once more in the saddle in search of further adventure. In "THE PONY RIDER BOYS IN THE OZARKS," they met with a series of disasters and exciting experiences which tested their courage almost to the breaking point. They were beset by a band of robbers, who stole their ponies. Nearly all the party, one by one, was lost in the fastnesses of the Ozark wilderness. It will be recalled how the boys, during a visit to the Red Star Mine, were caught in a wreck far underground; how a car of dynamite exploded, making them prisoners in a rocky tomb, and how, after being rescued by a mountain girl, they discovered the real secret of the Ruby Mountain, narrowly escaping with their lives in doing so. No sooner had they brought this eventful trip to a close than they set out to face the perils of the great, silent desert of Nevada.

They were almost upon it now. Its spell was upon them and the lads fell silent as they waited anxiously for the first sight of the land which they had journeyed so far to gaze upon.

They had not long to wait after leaving the water hole where they had replenished their supply.

The guide at last rode out upon a rocky promontory, where he halted, waiting for the others of the party to come up with him.

"Where's the desert—is that it?" demanded Ned, riding up beside him.

The guide raised his hand in a sweeping gesture.

"The desert lies before you," he answered, his eyes traveling meditatively over the miles of waste and mottled landscape.

A brazen glare lay over the scene, while up from the white alkali flats rose a wave of heat that was suffocating. Old, dried-up water sinks lay white and glistening here and there, framed by vast areas of sage brush, while on beyond in the blue distance lay miles and miles of monotonous, billowing hills and mountains.

"Whew!" gasped Chunky, mopping the perspiration from his brow. "This is somewhat hotter than Chillicothe, Missouri. I wish I had a cake of ice to put under my hat."

"Beautiful! Grand!" murmured Professor Zepplin.

"Reminds me of a Turkish bath I was in once in St. Louis," added Ned.

Tad Butler was silent. He was too profoundly impressed even to speak; and even the guide, familiar as he was with the scene, was silent and thoughtful, too. He understood full well the perils, the pitfalls for the unwary, that lay along the pathway of those who sought to traverse that barren waste.

At last he turned to Professor Zepplin.

"Shall we move?" he asked.

The Professor nodded.

"One of you boys get behind the burros and start them along, please," requested the guide.

Stacy Brown complied gleefully. No more pleasant task could have been assigned to him than that of prodding the lazy pack-bearers.

"Forward!" commanded Tom Parry.

The boys clucked to their ponies.

Not an animal moved.

Surprised, the lads brought their spurs against the flanks that they could feel were trembling a little.

A strange, unlooked for thing occurred.

With whinnies of terror the little animals reared and plunged. Before their puzzled riders could control them every pony in the outfit had whirled suddenly and began plunging along on the back trail.

A chorus of "whoa's" rose from the Pony Rider Boys. Quirt and spur were used freely, and firm hands on the bridle reins quickly checked the sudden rush. By dint of force and persuasion the boys finally succeeded in forcing their mounts back. That is, all had done so save Stacy Brown. His pony was spinning like a top, while Stacy red-faced and perspiring was uttering loud, angry shouts, driving in spur and raining quick, short blows on the animal's rump.

The burros had moved just far enough away to be out of reach of Stacy's plunging animal.

At last it threw itself violently to the ground. Stacy, by a remarkably lively jump, cleared his falling mount, but not a second too soon to save himself from being pinned beneath it.

He sat down on the animal's head, puffing from his exertions. After a minute, during which the other boys laughed so heartily that their own ponies nearly got the better of them again, Stacy rose and began prodding his mount with the end of the quirt, urging it to get up again.

But the pony refused to budge.

"He's 'hog-tied,'" nodded the guide, riding up. "Let him stay there till he gets ready to move. No use trying to hurry the beast. He's too much scared."

"Scared at what?" questioned Stacy, looking up apprehensively.

"Yes; that's what I'd like to know?" agreed Ned. "I don't see anything that looks like a scare."

The guide was looking down at the animal pityingly, Tad thought.

"What are they so frightened at, Mr. Parry?" asked the lad.

"My boy, they are afraid of the desert," replied the guide solemnly.

CHAPTER II

THE FIRST NIGHT IN CAMP

"The desert?" the Pony Riders gasped in chorus.

"Yes. It is not an uncommon thing. They seem to realize instinctively that there is danger off there. Even in animals that never have been near the desert you will find the same inborn dread of the alkali flats. And I don't know that I blame them any."

"But is my broncho going to lie here all day?" queried Chunky. "If that's his idea I might as well give him another argument that will make him change his mind."

"Let him alone. He'll be better off if you do not force him. When he gets up be gentle but firm with him."

"That's the strangest thing I ever saw," said Tad quietly.

"Most remarkable," agreed the Professor.

The faces of the boys were serious. They too