Ralph's exclamation had been caused by a peculiar substance with which his fingers had suddenly come in contact. Heretofore the walls had been rough, and in places rocky. Suddenly, however, his fingers encountered a rounded, smooth surface.
"What's the matter?" asked the professor, who was behind.
"I don't know. There's something odd imbedded in the wall right here. Can we spare a match?"
"I think under the circumstances we might," said the professor.
Walter accordingly kindled a fresh lucifer.
As its rays shone out, every one of the party shrank back with a cry of horror.
From the wall a grinning skull was gazing at them.
The ranch boy dropped his match with a cry of terror and startled alarm. Even the professor's nerves were shaken by this sudden apparition.
"F-f-for g-g-goodness' sake, strike another!" stuttered Ralph.
With trembling hands Walt struck another light, and this time they nerved themselves to examine the wall more carefully. The skull was imbedded in the rock, and by its side they now perceived was a skeleton hand, pointing down the tunnel. The professor also noted some marks at its side. There were five of them—short, straight lines, scratched in the wall.