He picked it up.

It was a human skull; but even as he lifted it it came apart in his hands and crumbled into fragments.

Freeman's nerves were strong, but he shuddered slightly.

The loneliness, the silence, the mystery, and the strange light-headedness that was coming over him combined to make him hesitate.
"I'll come back to-morrow morning early," he said to himself.
As if in answer, a deep, appalling roar broke forth apparently under his feet, and went rolling and reverberating up and down the canon.

It died away, but was immediately followed by another yet more loud, and the ground shook and swayed beneath his feet.