n 1822 by Captain Bell of the Minerva. I could study now
the madreporal system, to which are due the islands in this ocean.

Madrepores (which must not be mistaken for corals) have a tissue lined
with a calcareous crust, and the modifications of its structure have
induced M. Milne Edwards, my worthy master, to class them into five
sections. The animalcule that the marine polypus secretes live by
millions at the bottom of their cells. Their calcareous deposits become
rocks, reefs, and large and small islands. Here they form a ring,
surrounding a little inland lake, that communicates with the sea by
means of gaps. There they make barriers of reefs like those on the
coasts of New Caledonia and the various Pomoton islands. In other
places, like those at Reunion and at Maurice, they raise fringed reefs,
high, straight walls, near which the depth of the ocean is considerable.

Some cable-lengths off the shores of the Island of Clermont I admired
the gigantic work accomplished by these microscopical workers. These
walls are specially the work of those madrepores known as milleporas,
porites, madrepores, and astraeas. These polypi are found particularly
in the rough beds of the sea, near the surface; and consequently it is
from the upper part that they begin their operations, in which they
bury themselves by degrees with the debris of the secretions that
support them. Such is, at least, Darwin's theory, who thus explains the
formation of the _atolls_, a superior theory (to my mind) to that given
of the foundation of the madreporical works, summits of mountains or
volcanoes, that are submerged some feet below the level of the sea.

I could observe closely these curious walls, for perpendicularly they
were more than 300 yards deep, and our electric sheets lighted up this
calcareous matter brilliantly. Replying to a question Conseil asked me
as to the time these colossal barriers took to be raised, I astonished
him much by telling him that learned men reckoned it about the eighth
of an inch in a hundred years.

Towards evening Clermont-Tonnerre was lost in the distance, and the
route of the Nautilus was sensibly changed. After having crossed the
tropic of Capricorn in 135 deg. longitude, it sailed W.N.W., making
again for the tropical zone. Although the summer sun was very strong,
we did not suffer from heat, for at fifteen or twenty fathoms below the
surface, the temperature did not rise above from ten to twelve degrees.

On