"What surprises me," exclaimed Ned, "is the character of the opening. If it extended from cliff to cliff I should say that the same freak of nature that made this solitary island of rock also split off this end at some time. But it is closed at each end."
Alan hastened to the end of the fissure, near the side of the mesa.
"It looks to me," he said, "as if it had extended entirely across at some time and the ends walled up later."
The boys made a closer examination.
"You're right," said Ned when he discovered that each end of the rift had been filled with closely fitted rock, "and human hands did it."
Alan sprang up in excitement.
"That's the first sign we've had," he exclaimed. "Do you suppose it means anything?"
The edge of the cliff was so abrupt that the boys had to lie down to look over in safety.
"It does," Ned answered. "The reason you can't see that chasm from below or from in front is because the face of it is walled up. And it is walled so skillfully that you can't detect it from even a short distance."
"That's to hide something," quickly replied Alan, "but I don't see—"