What washes up on the shore of Lilliput?

Gulliver's hat washed up on the shore of Lilliput. the emperor of Lilliput was informed about it , he was told some of his subjects , riding near the place where first Gulliver was taken had seen a great black substance lying on the ground and had thus informed the emperor about it as by that 'things' size they concluded it to be something belonging to the man-mountain.