Still he dug down more and more till he had made a fair-sized excavation, when, once more thrusting in the spade, he found it checked against something, and his heart sank as he fancied that he might have struck upon a bed of old shells. Still he persevered, not that he expected to lift that which he touched, but in the hope that he might reach it more easily, and satisfy himself that he was touching metal. He was getting quite exhausted, and had already been down far too long. Nothing but the strong desire to have something definite to say kept him toiling on, and at last he unwillingly gave up, when something dark amongst the sand he had thrown out took his attention, and reaching down he picked up a lump of shells concreted together, and with an impatient gesture he was about to throw them down again, when it struck him that they were uncommonly heavy. To an inexperienced man this would have passed unnoticed, for the difficulty of telling the difference of weight in so dense a medium as the water was not one easily mastered, but Dutch had been down too many times not to have a good idea of such matters, and, checking himself just as he was about to throw the mass down, he raised it to the front of his helmet. Shells, shells, nothing but shells of several kinds joined together by the calcareous deposit of some kind of sea worm; but, all the same, it was very heavy, and, wrong or right, determining to take the lump up with him, he turned to go under the schooner and reach the ladder. For, he argued, those little ingots the Cuban had shown them had shelly accretion firmly attached, and it was probable that a good deal had been knocked off. At all events, he must ascend now, and going slowly along, placing the piece of concrete in a net pouch at his back, he was in the shadow of the schooner with its keel nearly above his head, when a peculiar sensation that he knew too well suddenly attacked him. His head began to swim, blood seemed to gorge the vessels of his eyes, and a horrible sensation of oppression to attack his chest. Already exhausted by his too long stay and extra exertion, combined with the nervous excitement of his fight with the shark, he was not master of himself, and in spite of his old experience he literally lost his head, becoming so unnerved that he sank down upon his knees, forgetting his signal-line, and tugging at the helmet to get it from his head.