SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. 105 
Messrs. Cornish and Kendall! have determined the relative solubility 
of aragonite and calcite organisms in water containing carbon dioxide, 
and their experiments show how readily aragonite organisms are dis- | 
integrated under these conditions. 
The great abundance of animal life in and around coral reefs might 
help to supply the carbon dioxide, which would be augmented on the 
death and decay of both animals and plants. In some such way as 
this, calcium carbonate in the rocks of the reef may be dissolved, and 
the condition for the deposition of the dissolved carbonate is the local 
removal of the carbon dioxide. It is not easy to explain how this re- 
moval can be effected, but it is possibly due to the presence in large 
numbers of excessively minute alge, whose absorption of carbon dioxide 
may help to bring about the precipitation of the calcium carbonate. As 
a rule, when calcium carbonate is deposited from cold water calcite alone 
is formed, while aragonite is deposited from hot solutions. But in coral 
limestones both calcite and aragonite are deposited from sea-water at 
the ordinary temperature, under suitable conditions. The physical and 
chemical properties of the organism on which the deposit is made, 
seem to have a determining influence on the particular form of calcium 
carbonate which is eventually formed. 
With regard to the mechanism of the chemical change from calcite to 
dolomite, very little is known. It would seem, however, that under 
certain conditions magnesium sulphate or chloride from sea-water can 
replace a certain amount of calcium carbonate in a limestone, and form 
the double carbonate dolomite. This point will be raised in the con- 
clusion, when discussing the results of Klement’s experiments. 
IV. Summary of Structural and Mineralogical Changes. 
In this part of the work an attempt has been made to trace the 
sequence of the structural and mineralogical changes which the rocks 
have undergone. This sequence is described as far as possible in the 
order in which the changes probably occurred. 
Under the microscope it is found that the rocks which have best 
preserved their original structures, and in which fewest secondary 
changes have supervened, are those from the fringing reefs of such 
islands as Niue, Vavau, and Christmas Island. 
A section of a coral from one of these reefs shows the centers of 
calcification and spicular structure well developed, while the cavities 
1 Geological Magazine, 1888, pp. 66-73. 
