"Why, I must be getting weak on my pins, for it seems as if the ground was trembling under me!" declared Roger, showing signs of sudden alarm. Dick and the guide exchanged hasty glances. Apparently they were feeling something of a similar nature, but could not lay it to the same cause as Roger. "It is getting much worse now," cried Mayhew, "and I can hear a terrible grumbling down underneath me that I must say I don't like over much!" All of them were by this time aroused to a sense of their sudden peril; but it was Dick who voiced the alarming truth. "Run for your lives!" he shouted, "it must be a boiling fountain about to burst, and we are right on top of the crater!" CHAPTER XXVIII WITH HOPEFUL HEARTS FORGOTTEN at that minute were all their other troubles, as each made hurried efforts to get away from the spot. The trembling of the rocks told plainly enough that some convulsion of nature was about to take place; and Dick's words gave evidence that he himself had discovered where the yawning crater of the boiling spring lay amidst the half melted snow. Hardly had they gone back some thirty or forty feet when there burst forth a vast volume of spray and steaming water that ascended high into the air, reaching an altitude of possibly five score feet before it lost its velocity, and began to rain downward. Immediately the snow around the entire vicinity commenced to melt with the heat of the falling water. With the sunlight falling on the wonderful fountain the two boys thought they had never in all their lives seen anything so sublime. Mayhew was more alarmed than pleased by the spectacle. Its terror appealed more to his backwoods nature than any beauty connected with the display. Indeed, had he been there alone, the chances were Mayhew would have taken to his heels, not being desirous of such close acquaintance with things he could not comprehend. After staring at the magical fountain for a brief time Roger once more allowed his other nature to have sway. "We are losing precious minutes, Dick!" he called out, for the roar was so great that it was impossible to be heard unless the voice were elevated. "This may be all very fine, but it doesn't seem to satisfy the gnawing sensation inside me." When Dick gave the word, Mayhew only too gladly once more led off. "That buffalo must have known of this hot fountain, because he came straight up to where it is," Roger remarked, after they had left the spouting geyser behind them. "Yes, but it seems that it rises only at long intervals," Dick explained. "It may be that days pass without an outburst. That accounts for the snow around, which disappeared so fast once the flow of hot water began."