: but their outsides run down, like a mountain wall,
sheer into seas hundreds of fathoms deep. People used to believe, and
reasonably enough, that the coral polypes began to build up the islands
from the very bottom of the deep sea.

But that would not account for the top of them being of the shape of a
ring; and in time it was found out that the corals would not build except
in shallow water, twenty or thirty fathoms deep at most, and men were at
their wits' ends to find out the riddle. Then said Mr. Darwin, "Suppose
one of those beautiful South Sea Islands, like Tahiti, the Queen of
Isles, with its ring of coral-reef all round its shore, began sinking
slowly under the sea. The land, as it sunk, would be gone for good and
all: but the coral-reef round it would not, because the coral polypes
would build up and up continually upon the skeletons of their dead
parents, to get to the surface of the water, and would keep close to the
top outside, however much the land sunk inside; and when the island had
sunk completely beneath the sea, what would be left? What must be left
but a ring of coral reef, around the spot where the last mountain peak of
the island sank beneath the sea?" And so Mr. Darwin explained the shapes
of hundreds of coral islands in the Pacific; and proved, too, some
strange things besides (he proved, and other men, like Mr. Wallace, whose
excellent book on the East Indian islands you must read some day, have
proved in other ways) that there was once a great continent, joined
perhaps to Australia and to New Guinea, in the Pacific Ocean, where is
now nothing but deep sea, and coral-reefs which mark the mountain ranges
of that sunken world.

But how does the coral ever rise above the surface of the water and turn
into hard stone?

Of course the coral polypes cannot build above the high-tide mark; but
the surf which beats upon them piles up their broken fragments just as a
sea-beach is piled up, and hammers them together with that water hammer
which is heavier and stronger than any you have ever seen in a smith's
forge. And then, as is the fashion of lime, the whole mass sets and
becomes hard, as you may see mortar set; and so you have a low island a
few feet above the sea. Then sea-birds come to it, and rest and build;
and seeds are floated thither from far lands; and among them almost
always the cocoa-nut, which loves to grow by the sea-shore, and groves of
cocoa palms grow up upon the lonely isle.