60-foot embankment down which he had slid.

"It's a wonder they wouldn't hang out a red light when they dig a hole like this," he declared angrily, "and not let a fellow most break his neck, to say nothing of scratching his eyes out! This is worse than a subway cave-in."

He pulled himself together and surveyed his surroundings.

The basin looked very much like an old quarry—so old that the shrubbery on the sides had grown into good-sized trees, and the whole place was covered with herbage of one sort or another. In one corner of the excavation, which must have covered some two acres, there was the ruin of an adobe house, while near the center was a stone structure made of four stone pillars about twenty feet apart and roofed over with two huge stone slabs, set so as to form a gable roof. Except for its size, it had the appearance of the old-fashioned well houses, which were once so common in New England.

"It's a tough-looking place, whatever it is," was Billie's comment. "I wish the fellows were here."

And then for the first time in more than half an hour Billie bethought him of his companions. His strange experience with the ape had driven all other thoughts out of his mind.

"By George!" he exclaimed aloud, "I wonder how the fight with the bandits came out?"

Almost as in answer to his words, there appeared upon the edge of the excavation into which he had fallen, but upon the opposite side from that on which he had taken his slide, ten horsemen, three of whom carried across the pommel of their saddles the bodies of three men. They halted and surveyed the basin critically. Then, single file, they slowly descended into the quarry.

Billie recognized them the minute he laid eyes upon them. They were the remnant of the bandit band, and the bodies carried across the pommels of the saddles were three of their wounded companions.

"This is no place for me," commented Billie as he kept himself well hidden behind a giant cactus. "It reminds me of Ali Baba and the forty thieves. I hope I have better luck than Ali Baba."

As though to carry out the trend of Billie's thoughts, the horsemen halted near the ruin of the adobe house before mentioned and two of their number dismounted and entered. A minute later the rest of the band rode into the ruins and disappeared, followed by the riderless horses of the two dismounted men.

Billie rubbed his eyes.

"I wonder if I'm getting dippy," he muttered. "Maybe that crack on the side of my head has made me see things."

He sat down to think.

"If I only had some kind of a gun," he mused, "I wouldn't feel so everlasting helpless. Confound that ape! If I ever see him again I'll break his neck."

Then, after a moment's thought: "I don't believe the beast would give up the chase. He's likely to show up at any minute. Something has to be done."

The boy scanned the edge of the embankment, if perchance he might see anything of his persecutor. There was nothing in sight and he decided to go on a tour of inspection. As quietly as possible he stole along the side of the excavation toward the spot where the ruin stood, when once more he had that sense of being watched.

Turning his head quickly, he saw the ape about twenty paces to one side aiming the revolver at him.

Then Billie lost his temper.

"This thing has got to stop," he exclaimed. "I'll be doggoned if I'm going to stand for it any longer."

He ran quickly toward the ape, and fell on his knees as he had seen the ape do, and raised his hands in supplication.

The animal quirked its head and fairly beamed with pleasure as it slowly advanced and stood beside him.

But its simian smile was quickly turned to surprise, for like a flash Billie snatched the weapon from its hand and aimed it at the animal's head.

"Now," he said, "you come with me."

The animal made no move.

"We'll see whether you will come or not," said Billie, and stooping down he broke off a good-sized sprout from a live oak. "Now, march!" and he raised the whip.

It was the one thing needed. The beast had evidently felt the touch of a whip before, for it raised its arm and danced about as though going through some circus maneuver.

"The first mystery is solved," laughed Billie. "Now for the second one. Come on, Ab," unconsciously naming his companion after the hero of Stanley Waterloo's famous story.

The ape seemed to know what was wanted and the two proceeded slowly and silently toward the ruin.

"I've been in a whole lot of queer scrapes," mused Billie as he crept along, "but this is surely the queerest—tramping around with an ape to solve the disappearance of ten cutthroats. I hope I wake up pretty soon."

But it was no dream, as Billie was soon to discover.

Arriving at the ruined building, Billie crawled along by the wall until he came directly under what had once been a window. Then, after listening a long time and hearing no sound, he ventured to raise his head and peep in.

The old ruin was as empty as though there were not a living person within fifty miles.

"Great Scott!" gasped the boy. "What do you think of that! Now I know I'm dreaming!"

He turned to the ape, waving his whip.

"Here you, Ab," he said, "go in there and see what you can find."

He pointed to the window and the animal sprang lightly in and a minute later perched itself on one of the decaying rafters.

"If there were any one around, they would certainly see Ab," reasoned Billie, "and would make some noise about it. I guess it's safe to go in."

He crawled around to the door and entered. There were the tracks of the horses, but the horses had disappeared as completely as though they had been swallowed up.

Billie called softly to Ab, who did not see fit to obey until he waved his whip. Then the animal sprang lightly to the ground. Billie showed him the tracks.

"Where do you suppose they went?" he asked.

Ab blinked his eyes and, for the first time since Billie had become acquainted with him, made a noise in his throat, much like the voice of a child.

Billie smiled in spite of himself.

"I'm glad you've decided to become sociable," he said, "What do you make of this? You look as though you could think."

Ab blinked his eyes stupidly and then suddenly became alert as though listening.

"What is it?" asked Billie, impressed by the ape's attitude.

For a reply Ab sprang through the window and made straight for the structure in the middle of the quarry. In another instant he was on the roof.

Billie followed as fast as he could and as he stepped beneath the roof started back in the utmost amazement, for up through what looked like a huge well there came the distinct sound of human voices.

For several seconds he stood as one in a spell and then he started forward to peer into the well, but on a second thought did not.

"I couldn't see anything if I did," he thought, "but any one looking up could see me. I'll do better by listening."

The words came to him almost as distinctly as though he were in the same room with the speakers, and there was no longer any doubt that the voices were those of the Zapatistas who had attempted to rob the train.

From their conversation Billie learned the outcome of the fight, and he was greatly amused at the attempt of the bandits to figure out who had betrayed them. Each one had a different theory, but all agreed that there must have been a traitor in the band. It was all the lad could do to prevent himself from calling out to them, just to see what effect his words would have.

After discovering from the