392 CORALS AND CORAL ISLANDS. 
the atmosphere; (2) the respiration of all the animal life in 
the waters, even down to the simplest and minutest; and (3) 
the decomposition of all vegetable or animal débris in the 
waters or diffused through the sand or muds. This gas is set 
free, therefore, just where it is needed for the work, and is 
always ready to perform its part in the process of consolida- 
tion. It enables the water to take up carbonate of lime from 
the grains of the mass to be solidified, or from outside sources ; 
and then the deposition of the same among the grains through 
their attractions produces the cementation. 
The beach and drift sand-rocks or odlites are different 
from the reef-rock in being superficial deposits. The carbonic 
acid of the waters performs the same part as in the latter; but 
with these, there is alternate wetting and drying during the 
ebb and flow of the tides and the succession of gales and quiet ° 
winds. By this means, the grains become incrusted, and every 
new wetting and drying adds a new layer to the surface of each; 
and thus the odlitic structure is produced. [acts are men- 
tioned on page 153 of pebbles of volcanic or basaltic rocks, 
lying loose on a seashore, becoming incrusted in this way with 
a milky layer; and of basaltic conglomerates being made by 
the same means, the carbonate of lime being added until all 
the intervals between the stones were filled up and the whole 
made solid; and of an amygdaloidal volcanic rock on a coast 
having derived its little calcareous kernels or amygdules from 
the same source. ‘The following additional facts are cited 
from Mr. Darwin’s Journal (p. 588): 
“ Lieutenant Evans informs me that during the six years 
he has resided on this island (Ascension) he has always ob- 
served that in the months of October and November, when 
the sand [of a calcareous beach] commences travelling to- 
ward the southwest, the rocks which are situated at the end 
