LYING about on the stone floor were at least ten human skeletons, which were white with age. They were in no fixed positions, but were scattered aimlessly over the dusty floor.

Bob was the first to break the silence.

“Wonder how they got here?” he asked.

“Perhaps they are the remains of soldiers who guarded this tower,” was the opinion of Dr. Rander. “Here,” he went on, “look at this. It’s a dagger—made of stone.”

“You might be right,” commented Joe. “But then, it’s possible that the dagger belonged to someone who came up and murdered those who were here.”

The explorers examined the skeletons for several minutes before going to the edge and looking down on the surrounding valley.

“Can see quite a distance,” observed Bob, peering out at a distant mountain.

“Yes,” agreed Joe. “There——”

At that moment he was interrupted by a cracking noise, which seemed to come from the tower.

“What’s that?” cried Dr. Rander excitedly.

But when it was repeated, there was no doubt in their minds as to the origin of the strange noise.

“It’s the tower!” exclaimed Bob fearfully. “I hope I’m wrong, but I’m afraid it’s going to fall. Hurry! Let’s get down to the ground.”

The three lost no time in scrambling down the stairs. They reached the bottom in safety, and then dashed outside, Dr. Rander displaying remarkable agility for his age.

Then something happened that filled them with terror. With one last creak, the old tower leaned slowly to the south, poised for a brief moment, and went crumbling to the ground. There was a report like that of a cannon, followed by a heavy cloud of dust and small particles of rock.

The explorers’ hearts were beating like trip hammers; their faces were red with excitement.

“Thank God that we escaped in time!” breathed Dr. Rander. “A few seconds more and we would have been caught beneath tons of rock!”

“And what a death!” muttered Joe with a shudder. “I don’t want to even think of it.”

They sat down on the ground to relieve themselves of the terrible strain. Especially did the old man show signs of exertion. At first the youths half expected to see him fall with heart failure, for he was terribly exhausted.

The rest, however, did him good, and before long he was apparently no worse for the horrible experience.

“Funny that tower should collapse just at this time, isn’t it?” said Bob. “I suppose, though, that it was ready to go at any time, and our weight proved too much for it.”

“That probably explains what happened,” came from the old man. “If no one had found it, perhaps it would have remained intact for many years yet.”

They put in no more time here, but turned and went back to the peak.

Very difficult it was to scale the steep slope, but at last they reached the top and descended to the other side.

“The mules are all right,” observed Bob. “Suppose we get going without delay. I’d like to get to Pasaje as soon as possible. I’m getting anxious to see Dad and the others of the expedition.”

“It will still take time,” Dr. Rander told him. “There are many miles of difficult travel before us.”

And difficult it was. Although the adventurers had previously found it hard to make progress through the rugged mountains, they now experienced even more hindrances.

Unfortunately, Dr. Rander was not overly familiar with this section and often made mistakes in pointing out the best course of travel.

It was not uncommon for the adventurers to come upon heavy forests at elevations of fifteen thousand feet. This surprised Bob and Joe, who did not expect to see trees growing at such high altitudes.

“There really is no such thing as the ‘tree line’ in the Andes,” explained the old explorer. “In fact, it is possible to find forests at unusually high altitudes.”

As they trudged on they came to more wild-cherry plants, the fruit of which they ate hungrily. It was refreshing to have fresh fruit after eating nothing but dried goods.

As much as possible they followed the banks of the Apurimac River, for Pasaje, their destination, is situated just above the point where the stream branches.

As they went farther, the mountains became more lofty, and snow on the summits was more common. They never climbed high enough, however, to come in contact with it.

Suddenly, upon hearing an unusual noise, Joe glanced around, and then gasped in apprehension.

“One of the mules is stuck in mud!” he cried. “Quick! We must do something at once!”

Without delay the old man turned about, walked back to one of the pack animals, and secured a long heavy rope. Then he moved on to the rear of the line, where one of the mules was struggling to free itself from the black bog.

“Bob, you had better come to the rescue,” he said, after a moment of pondering. “You’re pretty good at lassoing, as you showed us several days ago on that narrow ledge. Won’t you try it and see what you can do?”

“Where will I throw the rope, over the mule’s head?”

“Yes. Tighten it around the animal’s neck.”

“But—won’t it choke?”

“Not for a few minutes. There is no other way to pull the mule out. If we go too close we’ll get caught ourselves.”

“All right, then. Here goes.”

Bob took the rope, made a loop, and stepped as near as he thought possible with safety.

“If I miss my aim, I can’t help it,” the youth said, as he threw the lasso.

Joe cried out happily as he saw that the loop went directly over the mule’s head. Dr. Rander’s face also lightened.

All three pulled on the rope with all their strength, but their efforts appeared to be in vain. The treacherous bog had engulfed its helpless victim too tightly.

“We’ve got to get that animal out!” cried Joe, redoubling his efforts. “There’s a lot of valuable supplies on its back.”

But how? They were doing all they knew of. If that were not sufficient, the mule would have to go down.

“We’re losing steadily,” observed Bob grimly. “But I guess we can’t help it.”

Despite their furious efforts to draw the beast to safety, it was sinking rapidly. Already its body was nearly under. In but a brief time its head, too, would be engulfed.

But the adventurers kept up doggedly, determined to win the battle even though defeat was staring them in the face.

With one last effort, they gave the rope a new stronger pull. But it was not enough. The weight of the mule and its burden was too much, and the animal’s head slid horribly into the oozing mud, to disappear forever.

Bob slumped down on the grass.

“We lost,” he murmured gravely. “Now we’re one pack animal short.”

“And there were quite a few valuable objects in the pack, too,” added Joe, “not to say anything about losing the animal.”

“Have to get along some way,” Dr. Rander said. “There is no use in thinking anything more about it.”

“Wonder what the mule thought about when he went under?” mused Joe. “It must have been terrible.”

They resolved to follow the old man’s suggestion and forget the loss as best they could. After all, it was lucky that one of them had not been the victim.

Again they took up the journey, this time keeping a closer lookout for other bogs. But Dr. Rander did not know this region any too well, and could not guide them as surely as he would have liked to.

In the fascinating mountains the time passed rapidly. It was two days after they had lost the mule when Dr. Rander pointed to something in the distant sky.

“That’s a condor,” he said, his eyes trying to make out the flying form more clearly.

“It is at that,” affirmed Bob, looking through his binoculars. “And what’s more, it’s white. A condor real, as sure as I’m standing here!”

“Let’s see,” said Joe, and took the glasses his friend handed him.

A few moments of observing and he nodded, giving the binoculars to the old man.

“You’re right,” Joe said to his chum. “And oh! Wouldn’t our dads and Mr. Wallace like to be here now! A white condor! One of the creatures they wanted most.”

“Maybe we can get it for them,” muttered Bob, grasping a shotgun. “Do you suppose there’s a chance?”

“Not unless it flies nearer,” returned Dr. Rander. “Even then the bird would probably fall to the bottom of the caÑon, and that would mean a dangerous descent.”

“I’m going to see, anyway,” Bob persisted, following the soaring form through his powerful binoculars.

“Look what I’ve found,” called Joe, who had climbed down to a narrow ledge several yards below the others.

“Why—it’s a nest,” exclaimed Bob, upon descending to the shelf. “And look at those eggs. Sure are whoppers. Say,” he cried suddenly, “I bet I know everything now. This belongs to that white condor. Look. There’s a white feather. See it?”

“You’re probably right,” said Dr. Rander, moving down to the ledge. “Let us look about more. We may find something else of interest.”

But all searching was stopped a second later when Joe happened to glance up.

“The condor!” he cried fearfully. “It’s coming at us!”