When Frank had somewhat recovered from his astonishment, and his eyes had become accustomed to the dim light, he found the cay, or channel, to be some fifty yards in extent, cut through the soft, porous rock by the action of the water, that for ages and ages of time had beaten against its gradually-yielding base, until it had made for itself a passage such as man, with all his marvelous ingenuity, could never have fashioned. Frank rowed the entire length of the cay--as the Bay Islanders call these little wave-made inlets--coming out on the opposite side to that which he had entered; and then, as it was getting late, he returned home, as the brave-hearted boy termed the spot where he had pitched his tent and stored his provisions. Apart from finding the channel, he had made no discovery worth mentioning. With the exception of a few sea-birds, he saw no living creature, great or small; but this he did not much mind, for he hoped a sail would come his way soon, and solitude was no new thing to him. So he ate his supper with hearty relish, and, when it was dark, clambered into his hammock and fell peacefully asleep. CHAPTER XIII. A CHANGE OF PLANS. The morning of the tenth day of his residence upon the island Frank rowed around to the grotto--as he called his new-found giant's causeway--taking with him his fishing-tackle and a substantial luncheon of bread and cheese and dried beef. Fish of various kinds abounded in the quiet waters of the inlet, and in an hour he had caught as many as he wished to carry "home." He had seen no sharks anywhere near the reef, and so, when he saw a beautiful pearly-white shell lying at the bottom of the water, which was not more than five feet deep under any part of the natural arch of soft porous stone, he threw off his clothes and unhesitatingly made a dive for it. He got the shell, and made a very important discovery at one and the same time. Happening to glance upward as he came to the surface, his quick eye saw a low, narrow opening leading directly into what seemed to be the solid rock. The mouth of the cavern was slightly shelving, and situated a little less than mid-way of the centre of the arch. Frank lost no time in climbing into it, and was surprised to find himself in a semi-dark, sea-scented cavern, in shape something like an old-fashioned Dutch oven and fully seven feet in height. There was sufficient light to enable him to see that the floor of the cave was thickly strewn with fragments of shells and gray-white coral, the stone itself being so soft that he could easily penetrate it with his jack-knife. These submarine caves or grottos are numerous in the Bermudas, and the limestone rock of which they are mainly formed so extremely impressionable as to be readily cut into blocks for building purposes with a common saw.