He raised his hand to the aching spot and his fingers encountered a big lump.

The truth came upon him like a flash. He dropped his hand to his holster, and sprang to his feet.

As he did so he caught sight of the ape and found himself looking into the business end of his own weapon.

With a yell he dropped to the ground as though the expected had happened.

But when no shot followed, he began to regain his wits and lay still trying to figure out once more just how much the ape might know about the use of the weapon.

He remembered the old saying that a gun was a dangerous weapon without lock, stock or barrel, because a man killed his wife with the ramrod; and so he figured that an animal which had intelligence enough to throw a stone and knock him senseless, might have sense enough to fire a revolver.

"If I only knew something about his history," soliloquized Billie, "I might be able to guess how much he knew. But he is a perfect stranger to me. I don't even know his name."

After several minutes and nothing had happened, Billie decided to make some effort to get away.

"I might as well be shot as to be prisoner to an ape," he thought, and so he arose to a sitting posture and surveyed the scene.

There sat the ape as before, with the automatic pointed at Billie, but with a puzzled look upon its face. When the lad finally arose, the ape appeared still more puzzled and at length, turning the weapon away from Billie, looked into the muzzle.

"That settles it," exclaimed Billie. "He doesn't know how to fire it. I'll go and take it away from him."

He started toward the animal, which at once pointed the revolver in Billie's direction. There came a sharp report and a bullet whizzed by the boy's head.

"Worse and more of it," exclaimed Billie. "He doesn't know how to use the thing, but he's liable to shoot me as long as I stay in range. I'll just make myself scarce."

Stooping down, he picked up a good-sized stone and hurled it at the ape and then, without waiting to see the result of his throw, jumped into the jungle which lined both sides of the track, determined to make a detour and if possible lose his unpleasant companion.

He had not run far before he realized that the ape was following, but this he did not mind. There were plenty of trees between them, and he felt sure he would soon be able to reach some sort of a habitation, when he suddenly found himself on the edge of a deep basin into which he plunged before he was able to gain his equilibrium.

CHAPTER IV.

THE ROSARIO VIEJO.

To be suddenly pitched head-foremost down a rocky declivity into a ma.s.s of p.r.i.c.kly pear bushes and other tropical brambles is by no means pleasant; and as a result Billie was not in the best of humor when he picked himself up and looked to the top of the 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 hors.e.m.e.n, 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 hors.e.m.e.n 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 s.n.a.t.c.hed 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 sc.r.a.pes," 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."