104 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
Allowing for the disturbing effect in the proximity of volcanic rocks, it 
will be noticed that absence of insoluble residue may be regarded as char- 
acteristic of coral limestones formed in shallow water and under oceanic 
conditions, while deep-sea limestones and fringing reefs to great land 
masses normally contain upwards of 1 per cent of insoluble residue. 
It seems possible that these facts may be found to be of use in inter- 
preting the mode of origin of some of the older limestones of the 
earth’s crust. 
The mineralogical and structural changes which take place in coral 
limestones are consequent upon certain chemical changes in the rocks, 
which may be briefly considered at this point. The formation of the 
investing fibrous deposit of calcite round organic fragments, noticed in 
rocks from some of the fringing reefs (Figure 3), is probably due to 
the coating of fragments with a deposit of calcium carbonate formed 
on a beach by the evaporation of sea-water on the recession of the tide. 
The carbon dioxide present in the water holds a certain amount of cal- 
cium carbonate in solution. This is precipitated when exposed to the 
air, owing to the evaporation of the water and the loss of carbon dioxide. 
A macroscopic study of the limestones also shows that a certain amount 
of solution must have taken place since the rocks have been upraised. 
It is noticed that many cavities in the limestones have been more or 
less filled with a stalagmitic coating of calcite, and in certain cases, 
with dolomite. These deposits are no doubt due to the solvent properties 
of rain water bearing carbon dioxide in solution, and the precipitation 
of the dissolved carbonates on the escape of the carbon dioxide. While 
subaerial changes such as these do undoubtedly occur, yet most of the 
chemical changes have probably taken place below the sea-level. 
A study of the thin sections of the limestones shows that aragonite 
may arise from the alteration of calcareous mud, which fills certain cavi- 
ties in the corals, while it is occasionally deposited in clear crystals 
directly from solution. Again, calcite may be formed by the alteration 
both of calcareous muds and of aragonite crystals, while under other 
circumstances it is found deposited directly from solution. 
These processes require that the calcium carbonate shall first be 
dissolved in the sea-water, and, secondly, that under suitable conditions, 
it shall pass out of solution, and form a crystalline deposit. The 
solubility of calcium carbonate in sea-water is extremely small, and 
has been variously estimated at from 1 part in 10,000 to 1 part in 
136,000. In all probability the solution is largely increased by the aid 
of carbon dioxide dissolved in the sea-water. 
