STRUCTURE OF CORALS. 117 



Now, to explain the existence of barrier reefs, which lie so far from 

 the land, and in very deep water, as well as of atolls, which have, 

 apparently no land at all near them, we have to look for some dis- 

 turbing cause which will account for their presence in depths quite 

 fatal to the existence of the polypes, for all must die below the level 

 necessary to secure light, heat, air, and food. This disturbance, Mr. 

 Darwin has convinced us, we must look for in the slow subsidence 

 of the crust of the earth. Corals can only live and flourish at 

 depths of from 20 to 30 fathoms, and such a depth implies " no very 

 great distance from land." Suppose, now, the bottom of the sea to 

 sink slowly. Such corals as were at the lowest, being deprived of 

 the necessary light, air, warmth and food, would die, and so not 

 increase laterally, or outwards, towards the sea. But such as were 

 near the surface would grow upwards, still trying to find subsistence 

 by conformity to the new relations ; so that the upper edge would 

 still remain awash with the sea, and thus flourish. Now, during 

 this slow subsidence, the shelving coast line would evidently seem to 

 be removed further from the reef by its submergence, and thus the 

 distance of the barrier reef from land is explained. The same 

 reasoning as easily accounts for the formation of atolls, since an 

 island thus slowly subsiding must eventually disappear, leaving only 

 the encirling reef to mark its position. We see, then, in each 

 barrier-reef, a proof that the land has there subsided, and in each 

 atoll a monument over an island now lost. The very deep and 

 abrupt soundings outside both are thus also explained by the 

 death and non-growth of the lowest corals. The lesser depth 

 inside the reef is explained by the violent action of the tremendous 

 surf in those latitudes, by the action of which masses of coral are 

 broken off, carried over and inside the edge, and finally converted 

 into coral sand and mud. Smaller and more delicate corals also 

 grow inside, and help the process of diminishing the depth. 



One curious fact now remains to be noticed, namely, that in most, 

 if not all of these reefs, if a well be dug, it will be found to contain 

 nearly fresh, or very slightly brackish water, which makes them fit for 

 the habitation of man. These wells ebb and flow with the tides, and, 

 according to Mr. Darwin, " are common on some of the low islands 

 in the West Indies." The compressed sand, or porous coral rock 

 is permeated like a sponge with the salt water ; but the rain which 

 falls on the surface, must sink to the level of the surrounding sea, 

 and must accumulate there, displacing an equal bulk of the salt 



