Give a description of Gulliver's hat given by the Lilliputians.

From the text:

". . . a messenger arrived to tell his Majesty that some of his subjects, riding near where I was found, had seen a great black substance lying on the ground. It had wide, round edges, rising up in the middles as high as a man. It was not a living creature, as they thought at first. It did not move. Some of them walked around it several times, and climbing to the top and stomping on it, they found it was hollow. They thought it might be something belonging to the Man Mountain."