the others, was by this time on his feet.

"Matter?" howled Pete. "Matter enough. I do begin to think this place shore is haunted, or suthin'. As I lay there, I felt suthin' tiptoeing about behind me, and when I whipped suddenly round ter see if one of the critters hadn't broken loose, what did I see but a great, big, enormous thing, as big as a house, looking down at me. Afore I could say a word, it was gone."

"Gone!" echoed the others. "What was it?"

"Wish you'd tell me," sputtered the cow-puncher, looking about him, and still gripping his gun, "I never saw the like in all my born days."

"Well, what did it look like?"

"Hard to tell you," rejoined Pete. "It was as big as that." He pointed right up at the moon.

"As tall as the moon? Oh, come, Pete, you had dropped off and were dreaming," laughed Ralph.

"Who said it was as tall as the moon?" demanded the excited cow-puncher angrily. "I only meant to convey to your benighted senses some idee uv what it luked like."

"Well, how high was it?" asked Jack, in whose tones was a curious note of interest, for a reason we can guess.

"About twenty feet, as near's I could judge. It had red eyes, that glared like the tail-lamps of a train, and it spat fire, and it——"

"Whoa! Whoa!" laughed Walt Phelps. "Now we know it was a nightmare, Pete. The dream of a rarebit fiend. You ate too much crackers and cheese at supper."

"How was it we didn't see it?" asked Ralph, who had not spoken up till now.

"Why, you were lying with your back toward the direction it came from," explained Pete.

"An interesting optical delusion," declared the professor. "I must make a note of it, and——"

"Wow! There it goes ag'in."

"Where? Where?" chorused the boys.

"Right off there! Look! Look!"

The lanky cow-puncher, fairly dancing about with excitement, pointed out beyond the shadow of the solitary mesa. Sure enough, there were three or four enormous, black, shadowy shapes, traveling across the sands at a seemingly great speed.

"Get your rifles, boys!" yelled Jack.

The weapons lay handy, and in a jiffy four beads had been drawn on the immense, vague shapes.

But even as their fingers pressed the triggers, and the four reports rang out as one, the indefinite forms vanished as mysteriously as they had appeared.

CHAPTER IV.

SOME QUEER TRACKS.

The hour, the surroundings, and the utter mystery of the whole affair combined to make the sudden appearance and vanishment of the great shadowy shapes the more inexplicable, not to say alarming. Small wonder was it that the inquiring faces that turned toward each other were a trifle whiter than usual.

"What do you make of it, Pete?" asked Jack.

"Stumped, by the big horn spoon!" was the expressive response.

"No doubt, some natural phenomena, with a simple explanation," came from the professor. It was noted, though, that his angular form seemed to be somewhat shivery as he spoke, and that his teeth chattered like dice rattling in a box.

"Natural phe-nothings!" burst out Pete. "The things, whatever they was, were as solid as you or me."

"How was it they didn't make any noise, then?" inquired Ralph, practically.

"Waal, son, you jes' take a run on the bunchgrass, and you'll see that you won't make no racket, nuther."

Ralph did as he was directed, and it was really wonderful how silently he sped over the springy vegetation.

"Maybe it was somebody putting up a scare on us," suggested Walt, rather lamely.

"They couldn't rig up anything as big as that," said Jack decisively, "besides, there's another thing—I didn't tell you because I thought I might have been mistaken, but I saw those same things this afternoon."

"What?" went up in a perfect roar of incredulity.

"Say, is this some kind of a josh?" asked Coyote Pete suspiciously.

"Never more serious in my life," Jack assured him, and then went on to relate the strange experience that had befallen him when he had poked his head out from under the saddle in the sand-storm.

"If they weren't so enormous, I should say they was horses," said Pete; "but the biggest horses that ever growed never even approached them critters—spooks, er whatever they are."

"There are giants among men," suggested Walt, "why shouldn't there be giants among spooks, too?"

"You get to Halifax with that spook talk," said Coyote Pete scornfully. "I'll bet my Sunday sombrero that whatever them things is, there's some sore of human mischief back uv it. But what is it? Who put it up?"

"Yes, and what for, and why?" laughed Jack. "I tell you, fellows," he went on, "it's no use of our racking our brains to-night over this. The best thing we can do is to set a watch. Then, if they come again, we can try a shot at them. If not, why then in the morning we'll make an investigation; eh?"

"Durn good advice," grunted Coyote Pete. "Now, I'd suggest that ther perfusser takes ther fust watch, and——"

"No, no, my dear sir; really, I—I have a cold already. A-hem—ach-oo!"

The man of science, it seemed, had really developed serious bronchial trouble in record time.

"Why, professor," said Jack mischievously, "haven't I heard you say that you'd like a chance to investigate such a phenomenon as this?"

"Hum, yes—yes, my young friend. I may have said so, yes. And any other time I should be only too pleased to—Good Land! what's that?"

With the agility of a grasshopper, the professor had jumped fully three feet, as one of the pack-burros, nosing about behind him, accidentally butted him in the small of his back. The others burst into a roar of laughter, which they could not check. The professor, however, adjusted his spectacles solemnly and looked about him with much dignity.

"I thought I saw a book I had dropped, almost in the fire," he explained glibly, "so I jumped to get it before a hot ember fell on it."

"I had no idea you could jump like that, professor," laughed Jack. "You should have gone in for athletics at Stonefell."

It was finally decided that Walt and Ralph should stand the first watch, and Coyote Pete and Jack the last part of the night. The professor, after carefully drawing tight the curtain of his tent, "to keep the cold out," as he explained, retired. Soon after, Jack and the cow-puncher also went to bed till the watch should summon them to go on duty in their turn.

But the night passed without any reappearance of the strange shapes which had so upset the tranquility of the little camp, and, viewed in the fresh light of a new and glorious day, somehow the affair did not seem nearly so ominous and awe-inspiring as it had the night before. Breakfast, as you may imagine, was speedily disposed of, and, having seen to the stock, the party started out to explore the mesa itself.

As has been said, the side upon which they had camped the night before was nothing but a sheer cliff. Under the guidance of Coyote Pete, they now set out to encircle the strange precipitous formation. Their hearts beat high, and their eyes shone with an aroused sense of adventure as they strode along.

The professor carried with him a small volume containing a partial translation of the symbols and sign language of the ancient tribe whose domains they were about to invade. Jack had a coil of stout, half-inch manila rope, about two hundred feet in length. Walt Phelps' burden was a shovel, while Ralph Stetson carried an axe. All bore with them their revolvers, and Coyote Pete carried, in addition, a rifle.

"Are you afraid of anything?" the professor had asked him, as he noticed the sun-bronzed plainsman pick up this latter weapon.

"Waal," Pete had rejoined, with a portentous wink at the boys, "you never kin tell in this wale of tears what you're a-goin' up aginst—queer shapes, fer instance."

As they strode along, naturally the subject