At this Frank, who loved Mark as a brother, gave a shiver and crawled to the edge. But there was nothing below but dirt and tropical vines, the latter overrun with big spiders.
“Mark! Mark!” he shouted, and again the others joined in the cry.
“Hark! I heard something!” exclaimed Sam, who had walked nearest to the rough rocks where Mark had first slipped. “Listen.”
All did so, with bated breath. A low groan reached their ears, sounding as if it had come from the bowels of the earth.
“It’s Mark! He’s surely had a fall!” gasped Frank. He raised his voice: “Mark! Mark! Where are you?”
“Here, under the cliff,” was the faint answer, and another groan followed.
“Under the cliff?” repeated Professor Strong. He crawled to the edge and looked around as Frank had done. “I see no opening, do you?”
“No, sir.”
“The cries come from further up the cliff, in that direction,” said Sam, whose ears seemed to be more acute than the rest. “Listen! I think he must be in some hole over yonder.”
The professor hurried toward the rough rocks and was soon climbing around them. But he was more careful than Mark had been and made the turn in safety. He now found himself on another portion of the cliff and Mark’s groans came from directly beneath him.