"I think there are caves all over, or rather, under this country that the Indians call Kaintuckee," he said, "but down in this part of it they're the biggest." "You are right about Kentucky being a cave region," said the schoolmaster, "I think most of it is underlaid with rock, anywhere from five thousand to ten thousand feet thick, and in the course of ages, through geological decay or some kindred cause, it has become crisscrossed with holes like a great honeycomb." "I'm pretty sure about the caves," said Ross, "but what I want to know is about this peter dirt." "We'll find it and plenty of it," replied the master confidently. "That sample was full of niter, and when we leach it in our tubs we shall have the genuine saltpeter, explosive dust, if you choose to call it, that is the solution of gunpowder." "Which we can't do without," said Henry. They passed out of the Barrens, and entered a region of high, rough hills, and narrow little valleys. Hills and valleys alike were densely clothed with forest. Hart pointed to several, large holes in the sides of the hills, always at or near the base and said they were the mouths of caves. "But the big one, in which I got the peter dirt is farther on," he said. They came to the place he had in mind, just as the twilight was falling, a hole, a full man's height at the bottom of a narrow valley, but leading directly into the side of the circling hill that inclosed the bowl-like depression. Henry and Paul looked curiously at the black mouth and they felt some tremors at the knowledge that they were to go in there, and to remain inside the earth for a long time, shut from the light of day. It was the dark and not the fear of anything visible, that frightened them. But they made no attempt to enter that evening, although night would be the same as day in the cave. Instead they provided for a camp, as the horses and a sufficient guard would have to remain outside. The valley itself was an admirable place, since it contained pasturage for the horses, while at the far end was a little stream of water, flowing out of the hill and trickling away through a cleft into another and slightly lower valley. After tethering the horses, they built a fire near the cave mouth and sat down to cook, eat, rest and talk. "Ain't there danger from bad air in there?" asked Ross. "I've heard tell that sometimes in the ground air will blow all up, when fire is touched to it, just like a bar'l o' gunpowder." "The air felt just as fresh an' nice as daylight when I went in," said Hart, "an' if it comes to that it will be better than it is out here because it's allus even an' cool." "It is so," said the master meditatively. "All the caves discovered so far in Kentucky have fresh pure air. I do not undertake to account for it."