One hot afternoon at the end of a week found our canoemates half-way up the reef, and approaching a key called Lignum Vitæ, which is for several reasons one of the most remarkable of all the keys. It is a large island lifted higher above the surface of the water than any of the other keys, and it contains in its centre a small fresh-water lake. It is covered with an almost impenetrable forest growth, and concealed by this are ancient stone walls, of which no one knows the origin or date.