These long tramps and climbs in the open air had the effect generally of making the night's rest seem astoundingly brief to Jack, who lay down, be the bed hard or soft, took a few deep breaths, and then all was oblivion till it was time to rise. And it was so here high up on the mountain slope, upon a bed of soft grey ashes, with a thin canvas loosely hung tent fashion. One minute he was awake, thinking of the coolness of the wind at that height, the next fast asleep, and then, so it appeared, directly after staring at Ned, who had shaken him to announce breakfast, while a scent of newly-made coffee floated in through the opening in the canvas. In half-an-hour they were climbing the yielding slope of ashes diagonally, with the sun just appearing at the edge of the sea, glorifying the mists and the island below in a way that forced them to halt and gaze in wonderment at the beauty of the scene. Then up and up once more, but so slowly that a good two hours were spent over what had seemed to be to Jack the work of a few minutes. For the sides were deep in cindery stones which gave at every step, and ran down in little avalanches, leaving beneath a bed of fine silvery ash into which their feet sank deeply. To have gone straight up would have been impossible, but by a side movement the way was slowly won, and at last Jack paused for a few moments to get his breath, then hurried up the rest of the way, reached the top first, and was seen by those below to pass over what seemed to be a sharp edge and disappear. Ned shouted in alarm and rushed up after him, Sir John and the doctor next, for there was no reply to the shouts, and as just then a puff of smoke suddenly shot into the air, a horrible dread assailed the little group. But when they reached the edge they saw that their alarm was needless, for Jack had dropped into a sitting position upon the soft ashes, and was gazing down into a great cup-like depression about half-a-mile across, and gradually dipping down till the centre of the hollow was about five hundred feet below the top. "Not much to see, Ned," said Jack as the man joined him. "That must be where the bright glow comes from at night." He pointed down over the dark silvery grey waste, dotted with stones of all sizes, to where a pool lay on one side, apparently of water, for a shimmering light played over it, and a faint mist was rising slowly into the air. "Couldn't come from water, sir," said Ned. "I didn't expect to see a pond up here; but I suppose it's hot, and that's steam." "Oh yes, that's hot enough," said the doctor, who was panting with his exertions. "Liquid fire, eh, Jack?" "Wouldn't it be molten metal of some kind, father?" cried the boy. "No, my lad, it is molten stone--rock. Lava." "But it puzzles me," cried Jack, "how stone can melt. You said something to me one day about a flux."