284 
APPENDIX II. 
probably, of about 100 fatlioms) teems with organisms, 
calcareous and siliceous; such as algte, protozoa, liydrozoa, 
mollusca and other members of the animal kingdom : these 
are drifted by the currents from place to place ; by these 
the reef-building corals are supplied with food. It has been 
estimated, as the result of experiment, that a mass of 
ocean water one mile square and 100 fathoms deep con- 
tains more than sixteen tons of carbonate of lime. 1 After 
death the * skeletons ’ of these organisms are showered 
down upon the bed of the ocean. In water which exceeds 
some 800 or 900 fathoms in depth their remains are more 
or less affected by the solvent power of the carbonic acid 
present in the water, but at less depths they accumulate. 
Thus any submarine bank which rose within the above- 
named depth would be brought nearer to the surface, and 
its upper part, as the water above it shallowed, would be 
colonised by larger pelagic organisms ; these, after death, 
would augment by their remains the increasing pile of 
material, which at last would arrive within the bathy- 
metrical zone in which reef-building corals can live and the 
formation of an atoll would commence. 
As already pointed out by Mr. Darwin, 2 the corals on 
the outer margin of a bank grow vigorously, while the 
diminution of food and the increase of sediment tend to 
check the development of those in the inner part. Thus, 
1 I estimate that this amount of carbonate of lime is equivalent 
to a solid layer of the same area which is approximately '00009 of an 
inch thick. We may arrive at it thus : taking 2'7 as the specific 
gravity of carbonate of lime, we shall find the volume of sixteen tons 
to be about 212-4 cubic feet, or 7'S cubic yards. This has to be spread 
out over 3,097,600 square yards (the number of square yards in a 
mile), giving the above result. Even if we make a large allowance 
for the fact that the carbonate of lime is not solid, but in the form 
of an aggregate of hollow shells, I believe '09 of an inch is in excess 
rather than in defect of the truth. 
2 Page 87. 
