3\() STRUCTURE OF AN INDIVIDUAL REEF. 



dividual masses of species, which are either not 

 found elsewhere, and consequently never seen 

 alive,* or which greatly surpass their brethren on 

 other parts of the reef in size and importance. If 

 we approach the lee edge of the reef, either by 

 wading or in a boat, we find it covered with living 

 corals, commonly ina?andrina, astraea, and madre- 

 pore, in about equal abundance, all glowing with 

 rich colours, bristling with branches, or studded 

 with great knobs and blocks. When the edge of 

 the reef is very steep, it has sometimes overhanging 

 ledges, and is generally indented by narrow winding 

 channels and deep holes, leading into dark hollows 

 and cavities where nothing can be seen. When the 

 slope is more gentle, the great groups of living 

 corals and intervening spaces of white sand can be 

 siill discerned through the clear water to a depth 

 of forty or fifty feet, beyond which the water recovers 

 its usual deep blue. A coral reef, therefore, is a 

 mass of brute matter, living only at its outer surface, 

 and chiefly on its lateral slopes. It is believed that 

 coral animals cannot live at a great depth ; that 

 twenty, or at the most thirty fathoms is their ex- 

 treme limit of growth. This is apparently proved, 

 or nearly so, with respect to all known species of 



* I have seen a block of mteandrina, of irregular shape, 

 twelve or fifteen feet in diameter, the furrows of which, though 

 much worn and nearly obliterated, were wider than my three 

 fingers; also very large blocks and crags of a pontes, twenty 

 feet long and ten feet high, but all one connected mass, without 

 any breaks in its growtb. 



