It may be asked, With what rapidity are these coral and madreporic banks formed, so as to become _atolls_ and _fringing reefs_? To answer this question even approximately is very difficult. On the coast of the Mauritius, according to M. d'Archaic,[6] the learned professor of the Jardin des Plantes, the edge of the reef is produced by _Madrepora corymbosa_, _M. pocillifera_, and two species of _Astrea_, which pursue their operations at the depth of from eight to fifteen fathoms. At the base is a bank of _Seriatopora_, from fifteen to twenty fathoms in height. At the bottom, the sand is covered with _Seriatopora_. At twenty fathoms we also meet with fragments of _Madrepora_. Between twenty and forty fathoms the 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. _Millepora alcicornis_ extends from the surface to the depth of twelve fathoms; the _Madrepores_ and _Seriatopores_ down to twenty fathoms. Considerable masses of _Meandrina_ have been observed at sixteen fathoms; and a _Caryophylla_ has been brought up from eighty fathoms in thirty-three degrees south latitude. Among the polyps 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, _Millepora_ 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 madreporic 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.