He had practically determined to return to the top, shift his ladder along, and once more make the descent, when he realized his effort would be wasted. A thick, broken shelf of the pitted tufa jutted many feet out above the cave, and even beyond the growing weed. Should he hang his ladder directly before the opening, he would find himself, when he came to its end, swung helplessly over the water. He could see distinctly where the final base of the wall projected into the tideway. It would certainly be no less than ten feet removed from the nearest point he could possibly reach by this particular method. It occurred to his mind he could lengthen his strands, drop himself off the ladder-end, and swim to the edge of the cave. But, even as he turned to examine the physical features afforded to a swimmer, a huge dark form loafed like a shadow through the crystal tide, to rise beyond and cut the sparkling surface with a blackish dorsal fin. There was no mistaking Mr. Shark. Grenville nodded, grimly. "Thanks for the timely suggestion," he said, as the monster once more sank. He presently added, "It's a boat or no explorations." Somewhat disappointedly, he returned up the ladder to the top. "The cave is there," he told Elaine, who promptly sat down, in sheer relief, when she saw him finally safe, "but it has to be entered from the water." "Oh!" said Elaine. "But why does it have to be entered?" "Well," said Sidney, at a loss for a better argument, "it might be full of treasure;" and he smiled. Elaine was no less ready with her answer. "Treasure is certainly indispensable to us here. No wonder we've felt that something was strangely lacking." "There you are," he rejoined. "I think I can paddle the raft about the cliff, for the tide could never be better." She was certain that Grenville attached some unusual importance to an inspection of the cave. "Couldn't I help?" she asked him. "What was the fault of the ladder?" "Fully six feet too short. Perhaps you'd better watch for passing steamers. If we missed one--whom should we blame?" They had slowly returned to the shelter, where the table was attractively spread. "What a luncheon!" said Grenville, enthusiastically. "I'll eat in a rush and be back before you know I've gone." He certainly ate with lively promise. But, after the raft was launched on the tide, he lost all sense of time. He had left his shoes and stockings on the shore. He had brought a torch, lighted, which he lashed in an upright position on the raft. Wading and paddling, punting, pulling, and at times even pushing his craft along the beach, he warmed to his work in the briefest space, since the tide could hardly have been more favorable to his needs.