coral where the work of growth goes on. The inner or dead portions constitute the stony mass on which new material is secreted, but is no more essential for that purpose than a rock or a sandy shore. Prof. Dana observes that, if the living portion could be separated, it would form a hemispherical shell about half an inch thick. As the higher orders of trees increase in size by additions of new wood at the outer margin of the trunk, long after the heart-wood is dead, so the coral is alive and grows only on its surface.
It is obvious that the increase of the coral will continue without
Fig. 11.
Alcyonoid Polypes; "gayest and most delicate of coral shrubs."
limit except from surrounding conditions. Thus, if the dome reaches the surface of the water, the polypes die, and growth ceases in that direction; but it may increase in diameter, forming some remarkable structures, which we will presently notice. But, if the coralline mass be continually sinking by a subsidence of the land on which it rests, the conditions of growth continue, and reefs of tremendous mass and thickness are formed.
The dead coral is always more or less porous, until the pores and polype-cells are filled by comminuted substance or the infiltration of carbonate of lime. Aided by chemical changes, the mass becomes solid coral-rock, and finally compact limestone, with few traces remaining of
