178 
THE OCEAN WORLD. 
height. At the bottom, the sand is covered with Seria lojtorit. At 
twenty fathoms we also meet with fragments of Mculrepora. Between 
twenty and forty fathoms tire bottom is sandy, and the sounding-rod 
brings up great fragments of Caryophylla. According to MM. 
Quoy and Gaimard, the Astreas, which, as these naturalists consider, 
constitute the greater part of the reefs, cannot live beyond four or five 
fathoms deep. MiEepora alcicotnis extends from the surface to the 
depth of twelve fathoms ; the Madrepores and Seriatopores down to 
twenty fathoms. Considerable masses of Meandrina have been ob- 
served at sixteen fathoms ; and a Caryophylla has been brought up 
from eighty fathoms in thirty-three degrees south latitude. Among 
the polypiers which do not form solid reefs, Mr. Darwin mentions 
Cellaria, found at a hundred and ninety fathoms deep, Gorgonia at a 
hundred and sixty, Corallines at a hundred, MUltpora at from thirty to 
forty-five, Sertularias at forty, and Tubulipora at ninety-five fathoms. 
According to Dana, none of the species which form reefs — namely, 
Madrepora, Millepora., Porites, Astreas, and Meandrineas — can live at 
a greater depth than eighteen fathoms. It is only near the surface of 
the water that the zoophytes which produce minerals and form madre- 
poric banks put forth their powers ; the points most exposed to the 
beating of the waves is that which is most favourable to their growth , 
it is there that the Astreas, Porites, and Millepores most abound. 
The proportionate increase of the structures, according to Mr. 
Darwin, depends at once upon the species which construct the reefs and 
upon various accessary circumstances. The ordinary rate of increase 
of the madrepores, according to Dana, is about an inch and a half 
annually ; and, as their branches arc much scattered, this will not exceed 
half an inch in thickness of the whole surface covered by the madrepore- 
Again, in consequence of their porosity, this quantity will be reduced 
to three-eighths of an inch of compact matter. It is, besides, to be 
noted that great spaces are wanting; the sands filling up the 
destroyed part of the polypier are washed out by the currents in the 
great depths where there are no living polvpiers, and the surface 
occupied by them is reduced to a sixth of the whole coralline region; 
which reduces the preceding three-eighths to one-sixth. The shells 
and other organic debris will probably represent a fourth of the tota 
produce in relation to polypiers. In this manner, taking everything 
into account, the mean increase of a reef cannot exceed the eighth oi 
