It narrowed again, and pierced decidedly upward through the bulk of the huge rock mass. Obliged at last to stoop too low for comfort, Grenville began to wonder if the thing would never end. It appeared to be exceptionally straight for a natural tunnel in volcanic rock, but Sidney began to realize its upward incline had rapidly increased. When he presently found himself enabled to stand once more erect, he paused to cast a light on the walls to confirm a new thought in his mind. He had finally remembered a feature long before noted on top of the terrace itself--the long straight crack through the massive tower of tufa and the "slip" that had once formed a shelf. Not without a certain sort of excited hope did he now discover unmistakable signs that some convulsion of the island had at one time actually parted the right-hand mass of rock from the larger portion on the left and permitted the former to drop. If this channel could only continue---- He went upward again, more swiftly, wondering thus belatedly how far he might have come and regretting he had not thought to pace the distance. Through a place ahead he was barely able to force his supple body. Then came another passageway that was not only narrow but low. Fragments of stone were likewise under foot, and the passage formed another angle. Beyond this turn he found himself confronted by more broken stone and a difficult ascent. But, toiling up there eagerly, he presently raised his eyes and beheld a bright white line, as narrow as a streak of lightning. It was simply a crack through a shattered bit of wall that closed up the end of the passage. It was daylight--sky--that he saw thus slenderly defined, and the man could have shouted in joy! He could not, however, escape to the outside world when he presently came to the wall. For all the fragments he loosened and threw back behind him, he could not open the exit, or even determine where it was. Only work outside could accomplish this end, and this he was wild to begin. About to turn back and hasten to the terrace, he realized instantly how utterly impossible might be the task of finding the place from without. But Elaine was doubtless on the terrace. If only his voice could be carried to her ears, she could mark the spot at once. But, although he called with all his lusty might, there was no response from the camp where Elaine was doubtless working. His torch was burning low, with the draught fanning constantly past him through the channel. It occurred to his mind to go back to Elaine and instruct her how she could assist him. He also thought to place his torch against the crack and permit its smoke to filter through and perhaps thereby blacken the fissure. Until he felt he must save what remained, to illumine his way downward, he burned the torch close to the rocks. And thus, when he came to the larger cave again, he was once more obliged to depart with not even a sight of the treasure. CHAPTER XXIX