294 CORALS AND CORAL ISLANDS. 
It is, hence, concluded by them that, under these conditions, 
the simple bank of growing corals may have a depression 
made at centre, which, as the process continues, will become 
a lagoon basin, and the reef, thereby, an atoll with its lagoon; 
that the atoll, so begun, may continue to enlarge through the 
external widening of the reef and the further action of cur- 
rent abrasion and solution within; or, in the case of fringing 
reefs, that the change may go on until the reef has become a 
barrier-reef with an inner channel and inner reefs. It is ad- 
mitted that subsidence may possibly have helped in the case 
of the deepest lagoons. 
Dr. Geikie expresses his opinion on the subject thus :— 
“ As the atoll increases in size the lagoon becomes proportion- 
ally larger, partly from its waters being less supplied with 
pelagic food, and therefore less favorable to the growth of 
the more massive kinds of corals, partly from the imjurious 
effects of calcareous sediment upon coral growth there, and 
partly also from the solvent action of the carbonic acid of the 
sea-water upon the dead coral.” The argument has been en- 
forced by observations on the solvent power of carbonic acid. 
But this is a point about which there is no ground for dispute. 
a. Mr. Semper gives examples of the effects of currents 
at the Pelew Islands, stating that by striking against or flow- 
ing by the living corals they make the reef grow with steeper 
sides and determine its direction, and urging that abrasion 
and solution have made, not only the deep lagoon-like chan- 
nels, but the deeper channels between the islands. He holds 
that in Kriangle, which he describes as a true atoll with no 
channel leading into the lagoon from the sea, that the lagoon 
may have been “the result of the action of currents on the 
porous soil during a period of slow upheaval.’* He says, 
1 Animal Life, pp. 269, 270. 
