But there was still the stone.

It was sandstone, not so very hard.
If he could cut grooves in it, he might be able to draw out bars, cement, and all.

He sprang down to the floor again, and was thinking how he should best set to work, when a groan drew his attention to his companion.
"You seem sick, friend," said he.
"Sick in mind," moaned the other.

"Oh, the cursed fool that I have been! It maddens me!" "Something on your mind ?" said Amos Green, sitting down upon his billets of wood.

"What was it, then ?" The guardsman made a movement of impatience.