The boiling grew less and less until finally it ceased altogether, nothing having appeared. “There you are,” said Charley, triumphantly. “Just as I said.” “Your idea is, I suppose,” said Dick, “that the water is running off into the underground lake?” “My idea is,” said Charley, “that under the mountain on the left here is a big cavern at a lower level than this lake and that between it and the lake is a deep hole. When the hole is full of water it discharges into the cavern gradually by a small outlet, when it gets down to a certain level the water of Izard Lake runs down into the hole until the equilibrium is restored.” “Right,” said Dick. “Just what I think, exactly.” “It’s too deep for me,” said Doctor Dan, shaking his head. “What, the theory or the hole?” laughed Dick. “Both. I should have to see the hole before I believed in it and I don’t understand the theory of all.” “If we could only stay here and watch we would find that the boiling takes place at regular intervals and continues just so long,” added Charley. “We can investigate this later on if you say so, Dick.” “By all means,” replied Dick, “but now we must be on the move. I shall never rest until I have found Clara Eglinton and I only hope Doctor Dan’s theory proves correct.” “About the horse?” replied Doctor Dan. “You will find it entirely correct. If I am not up on science I am on horses. Give that mare her head and she will take us to the place where they turned off with the lady sure; that is, if there is any such place. As I understand the situation, you are not actually sure that Mudd captured her at all.” “Well, I have only his word for it,” said Dick, “and that don’t amount to much, I own.” They now moved on. The mare led the way into the pass through which Dick had come, continuing along about half the distance to the trail up the mountain, when she suddenly turned and stopped short up against the solid ledge. “Well!” exclaimed Doctor Dan, “this is queer!” The place into which the mare had turned was a sort of niche in the rocky wall, crescent shaped and perhaps forty feet deep. There was no break anywhere and the rocks towered to a height of several hundred feet above their heads. “What’s the matter with the beast? What does she stop here for?” demanded Dick. But Doctor Dan could give no satisfactory answer to this question. He dismounted and made a long and careful examination of the place without discovering anything to explain the conduct of the mare. “It beats me,” he said at last, “but one thing is certain there is no way through that ledge.” “Perhaps they just halted here for some purpose or other,” said Dick. “I think we had better push on. The mare may give us another steer.” But the mare did nothing of the sort. They continued on to the trail and then down the mountain to the alkali plains.