I want to see if there is a trap-door into this affair.

Don't be afraid, Mrs.Cliff.

It's as solid as a rock." The captain did not hand up the lantern, but holding it carefully in one hand, he ascended the dome by means of the row of protuberances on the other side, and crouched down beside Ralph on the top of it.
"Oh, ho!" said he, as he moved the lantern this way and that, "here is a square slab fitted into the very top." "Yes," said Ralph, "and it's got different mortar around the edges." "That is not mortar," said the captain.

"I believe it is some sort of resin.

Here, hold the lantern, and be careful of it." The captain took his jack--knife out of his pocket, and with the large blade began to dig into the substance which filled the joint around the slab, which was about eighteen inches square.