OBJECTIONS TO THE THEORY OF SUBSIDENCE. 299 
of the waters. An elevation would only hurry the shallow- 
ing and end in emptying the lagoon. 
f. Erosion through solvent action is promoted by the 
presence in the waters both of carbonic acid and organic 
acids. The material within reach of the tides or waves ex- 
posed to this action is dead corals and shells, or their debris, 
and bare coral rocks, occurring over: (1) the outer region of 
living corals and for a mile or so outside ; (2) the shore plat- 
form and the reef, bare at low tide, on which there is com- 
paratively little living coral; and (3) the lagoon basin. 
There is nothing in the material within the lagoon to favor 
solution more than in either of the other two regions; in 
fact, the platform and bare reef are most exposed to the 
action because of the small amount of living corals over 
them. The outside waters take up what they can through 
the carbonic acid they contain, and supply thereby the wants 
of the lime-secreting polyps, shells, ete., and carry on the pro- 
cess of solidification in the debris; the same waters move on 
over the atoll reef and take up more lime as far as the acid 
ingredient is present ; and then they pass to the lagoon for 
work similar to that outside, with probably a diminished 
amount of free carbonic acid, on account of the loss over the. 
reef-ground previously traversed. 
The lagoon-basin is not, therefore, the part of the atoll 
that loses most by solution, any more than by abrasion and 
transportation. The outer reefs suffer the most; and yet, if 
the island is not subsiding at too rapid a rate, they keep ex- 
tending and encroaching on the ocean, instead of wasting 
through the drifting into the ocean at large of calcium car- 
bonate in grains and solution, and the shore-platform also 
preserves its unvaried level notwithstanding the daily sweep 
of the tidal floods, and the holes that riddle its outer portion. 
