SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. 125 
tion of the limestone by percolating water, formed the beds of phosphate 
now found capping these hills. The occurrence of beds of dolomite im- 
mediately below the phosphate seems to point to its formation in waters 
near the surface, possibly those of the lagoon. The occurrence of dolo- 
mitic rocks from the inland cliffs and terraces of Christmas Island and 
Eua seems to support this view, although these dolomitic limestones may 
be derived from the débris of the older rocks of the central nucleus of 
these islands. Mr. E. C. Andrews states, however, that the lower dolo- 
mitic limestones of Ngillangillah are from a fringing reef. This view is 
supported by the fact that the upper raised fringing reefs in the Red Sea 
are dolomitized._ It seems probable that the introduction of magnesium 
into the limestones does take place from the waters of the lagoon under 
certain favorable conditions. What these conditions are it is at present 
impossible to say with any degree of certainty. It is improbable that 
concentration to any marked extent can take place in lagoons unless they 
are entirely shut off from the sea. Before this question can be attacked 
successfully much fuller information must be obtained as to the chemical 
composition of lagoon muds, and analyses must be made of the waters of 
lagoons and those of the open ocean outside coral islands. It may be 
that the carbon dioxide liberated on the death and decay of both plants 
and animals helps to dissolve their calcium carbonate, which under these 
conditions may react with magnesium sulphate of the sea-water, and 
precipitate the insoluble double carbonate of calcium and magnesium, 
while the more soluble calcium sulphate, formed as a result of the reac- 
tion, could remain in solution. Whatever the conditions for the intro- 
duction of magnesium carbonate intv coral limestone may be, it seems 
probable, from the distribution of magnesium carbonate in upraised 
coral islands, that the introduction takes place at or near the surface 
of the water, and that a limestone exposed to suitable conditions for a 
sufficiently long time will become dolomitic. The occurrence of dolo- 
mitic limestones at several horizons in an island might be accounted for 
either on the theory of subsidence or of elevation, by changes in the 
rate of movement of depression or upheaval. An ordinary non-mag- 
nesian limestone might result from a somewhat rapid subsidence or 
elevation, while a constant and slow subsidence or upheaval might result 
in the formation of a completely dolomitized island. The extent to 
which a rock will become dolomitic must depend largely on the dura- 
1 Walther, J. Abhandl. math. phys. Koénigl. Sachs. Gesellschaft der Wissen- 
schaften Bd. xiv. p. 494, Hume, Rift Valleys of E. Sinai. Int. Geol. Cong. Paris, 
1900, pp. 32-40. 
