Definition of "copestone"

The universe is finished; the copestone is on, and the chips were carted off a million years ago.

1851 November 14, Herman Melville, chapter 2, in Moby-Dick; or, The Whale, 1st American edition, New York, N.Y.: Harper & Brothers; London: Richard Bentley, page 10

For small erections may be finished by their first architects ; grand ones, true ones, ever leave the copestone to posterity.

1851 November 14, Herman Melville, chapter 32, in Moby-Dick; or, The Whale, 1st American edition, New York, N.Y.: Harper & Brothers; London: Richard Bentley, page 159