OVER their shoulders the youths glanced up the mountainside and to their horror saw that the old man was right.

Far up the slope was a great mass of stones and earth, rolling down in a heavy cloud of dust. Every second saw the accumulation nearer and larger. In but a brief time it would be upon the little group below.

“We must do something—without delay!” Dr. Rander’s voice was cool and resolute. “The mules—they must be pushed out of the way!”

Acting on the instant, Bob and Joe and the old man worked like madmen to drive the pack animals over to a flat shelf that was but a few hundred feet away. Even this did not offer absolute safety, for the mass of rocks and earth might change its course and plunge down on the shelf.

“We’ll have to take a chance,” Bob panted. “Most likely over here we’ll be out of the danger zone.”

The flat stretch offered the only place of refuge. If the avalanche should change its course, the adventurers and their pack animals would surely be crushed.

There was a moment of fearful waiting, a brief period when the hearts of all stood still, and the huge mass of rocks roared past and on down the mountain slope.

“Thank goodness!” breathed Dr. Rander. “A narrow escape if there ever was one.”

Their eyes followed the plunging stone heap until, with a dull thud, it struck the bottom of the abyss, far below.

“If that had struck us we’d have been smashed to pulp,” muttered Joe with a shudder.

“Think of something more pleasant,” smiled Bob. “The treasure, for instance.”

That afternoon the three were struggling along the narrow path when suddenly Bob caught sight of something up on a high shelf.

“Look,” he said, pointing up to the flat stretch. “Inca ruins as sure as I’m born.”

“You are right,” Dr. Rander assured him. “That building is an ancient temple, where the Incas worshiped the sun god.”

“Sounds interesting,” Joe said eagerly. “Why can’t we go up there and explore the place?”

“Can if you want to,” the old man replied. “I have been up, but I will go again with you. Wait until I tie the mules.”

It was a difficult climb to the shelf. All were gasping for breath when they finally reached the ruins.

Those ruins the youths found interesting. The building was partially intact, being constructed of huge stone blocks cemented together with mud. It was shaped like a pyramid, the sides sloping up to form a point at the top. The entire north end was in ruins, but the other sections looked as if they had been built but recently.

“Let’s go inside,” suggested Bob, looking at the dark opening that once no doubt was closed by a door.

The others agreed, and, led by Bob, entered the building. As the windows were large and there was no covering over them, the adventurers could easily see about.

They were in a large room, over which was an unusually high ceiling. Several stone implements were scattered about; what they were doing in a temple, the boys could not guess.

“Perhaps this was meant for a storehouse or some other kind of building,” thought Bob, but did not say so. He did not wish to conflict with Dr. Rander’s opinion.

But aside from these few stone objects, there was nothing in the building that was of particular interest. Whether the structure had been deserted by the Incas, or whether someone had robbed it of its former contents, the youths did not know. Surely, if Dr. Rander was right in his belief that he alone knew of this trail, no one had been here.

They explored the building eagerly in search of anything else that might have escaped their view before.

“Ouch!” cried Joe, slapping his leg. “What was that that bit me?”

“Oh, I should have told you,” began the old man with a smile. “There are fleas here.”

“What th——” Joe was perplexed. “Fleas? What do they live on? There isn’t anything in these ruins——”

“That is out of my knowledge,” Dr. Rander said. “I never have found anything yet that they might eat. Unless it would be other fleas.”

There was a general laugh.

“Whatever it is, it’s enough to keep them alive,” observed Bob, scratching his arm. “At least as long as we stay they’ll be able to secure a good living.”

“Suppose we get out and don’t stay any longer,” was the suggestion made by Joe Lewis. “There isn’t anything here——”

“What’s that?” cried Bob suddenly, interrupting his friend.

Bob had heard a peculiar noise that caused him to turn about.

Again it came, a hideous hiss that made them jump in alarm.

“Snake!” pronounced Dr. Rander, and an instant later a long slim body unwound itself and made slowly toward the three explorers.

“It’s poisonous, all right,” observed Bob, noting the peculiar shape of the reptile’s head.

“And we haven’t a gun!” moaned Joe ruefully. “What will we do?”

“You mean we haven’t a rifle, but we have a gun.” Bob produced his revolver, which he aimed carefully at the horrible staring eyes.

“I won’t dare miss!” he thought, and then pulled the trigger.

The report of the pistol mingled with a sickening swishing noise as the snake lashed about in its death struggle.

“You got him, all right.” Joe felt a refreshing feeling of relief creep over him.

Bob nodded.

“Let’s get out of here,” he said in a determined voice. “There’s no telling how many more of these creatures we may see if we don’t.”

“I should have brought the movie camera,” laughed Joe, as they descended the hill. “That would have made a peach of a picture.”

“I’m glad you didn’t,” came from Dr. Rander. “Because the sight of those ruins might have inspired some scientists to come over this trail. And the secret treasure cave might be known to them—if they could open the hidden door.”

Dr. Rander, however, had not protested to the youths’ taking movies of the region they were passing through, even though there might be danger of someone from the United States coming in search of other ruins and treasure hideouts.

“I could not say anything against your doing that,” he had told them. “Especially since you were engaged by the motion-picture house.”

So they had exposed film whenever they thought it advisable and were well pleased with the results so far. All told, they had already cranked off more than a thousand feet of film, and fully expected to use that much more.

“These scenes ought to be interesting, too,” remarked Bob, untying the foremost mule. “I believe the Neuman Motion Picture Corporation will be well satisfied with them.”

“Let’s hope so,” Joe said. “The money we get from doing this is no small sum.”

The next day they plunged into a region that was wilder and more dangerous to penetrate than any they had been in before. The rugged Andes reached up on all sides of them, often forming narrow valleys that were shut off from the rays of the sun.

“Sure is cold tonight,” remarked Joe with a shiver.

They were camped beside a small spring that was fed by an underground stream. Darkness had fallen, and with it had come the chill of the Andean night.

Joe was resting peacefully beside the warm blaze of the campfire when suddenly he caught sight of two small fiery lights coming slowly at him.