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.