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 att.i.tude.

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 conversation that the bandits felt themselves perfectly secure where they were and that they were likely to stay there for some time, he finally decided to get back to the railroad and thence to Pachuca and give the alarm.

Acting upon the decision, he made his way across the basin to where he had slid down the embankment and slowly and laboriously climbed to the top, followed by Ab.

Retracing his steps, he soon reached the railroad and looking down the track toward Pachuca gave a shout of delight as he saw his three chums approaching, leading a fourth horse between them.

"Just in time," he said, as they drew near enough to exchange greetings.

"Just in time for what?" queried the others as one.

"To capture the bandits that you let escape."

"How do you know that we let any escape?" asked Adrian.

"I'm a mind reader."

The other three looked incredulous.

"Oh, it's a fact," declared Billie. "I can tell you all about the fight just as well as though I had been there," and he proceeded to prove his words.

"Some one told you," said Adrian.

"Sure," laughed Billie, thoroughly enjoying the perplexity of his companions. "My friend Ab. Come here, old man, and be introduced," and Billie waved his whip, which he still carried.

With a little cry the ape sprang to his side, an act which only added to the amazement of the others.

"Allow me," said Billie with mock gravity, "to introduce my friend Mr. Ab from No-man's-land. Ab, these are the rest of the Broncho Rider Boys. How would you like to join the company?"

Ab scratched his ear and looked wise.

"Oh, come," interrupted Donald. "Tell us what's up and what we must do to capture the rest of this band of cutthroats."

Thinking that he had carried the joke far enough, and realizing also that he might be wasting valuable time, Billie related his adventure, describing the place he had discovered.

"It is the Rosario viejo," said Pedro, as soon as Billie had finished his story.

"What's that?" asked Billie.

"An abandoned mine. It is called the old Rosario to distinguish it from the new Rosario, which is now one of the most valuable mines in this region. The station at Pitahaya was built especially to serve it."

"I see," said Billie. "Where is the new mine?"

"Down there," and Pedro pointed to a trail leading in the opposite direction. "It is just about as far from the station on that side of the railroad as the old Rosario is on this."

"Don't you think we had better summon some a.s.sistance from the mine workers?" asked Adrian.

"They may be in cahoots with the bandits," laughed Donald. "I have lost my confidence in about every one in this bandit-ridden land."

"I don't blame you," said Pedro, "and I hope the day will soon come when all this trouble will be over."

"Then you agree with me that we should tackle the bandits alone, do you?" asked Donald.

"It is the only safe way."