270 CORALS AND CORAL ISLANDS. 
coral island, the earlier steps in which are illustrated in the 
preceding figures. 
It is to be noted in this connection that if an atoll-reef is 
not undergoing subsidence the coral and shell material pro- 
duced that is not lost by currents serves: (1) to widen the 
reef; (2) to steepen, as a consequence of the widening, the 
upper part of the submarine slopes; (3) to accumulate, on 
the reef, material for beaches and dry land; and (4) to fill 
the lagoon. In regions of barrier reefs the inner channel 
may be a large receiver, like the lagoon of the atoll. But if, 
while subsidence is in progress, the contributions from corals 
and shells exceed not greatly or feebly the loss by subsidence 
and current waste, the atoll-reef, unable to supply sufficient 
debris to raise the reef above tide-level by making beaches 
and dry-land accumulations, would (1) remain mostly a bare 
tide-washed reef; (2) lose in diameter or size hecause the 
debris that is not used to keep the reef at tide-level is carried 
over the narrow reef to the lagoon by the waves whose throw 
on all sides is shoreward; (5) lose in irregularity of outline 
and thus approximate toward an annular form; (4) lose the 
channels through the reef into the lagoon by the growth of 
corals and by consolidating debris; and (5) become at last a 
small bank of reef-rock with a half obliterated lagoon-basin. 
Some of the islands of the equatorial Pacific in this last con- 
dition are described on page 198. | 
Moreover, the subsidence, if more rapid than the increase 
of the coral reef, would become fatal to the atoll, by gradu- 
ally sinking it beneath the sea. Such a fate, as stated by 
Darwin, has actually befallen several atoll-formed reefs of 
the Chagos Group, in the Indian Ocean (p. 192); one of 
them has only “two or three very small pieces of living reef 
rising to the surface.’ Darwin calls such reefs dead reefs. 

