SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. 103 
among the coral islands of the Pacific. At Christmas Island, the sum- 
mits of Murray, Phosphate, and Ross Hills, according to Dr. C. W. 
Andrews, were low islands rising from the lagoon during Miopliocene’ 
times, and were covered with guano deposits. Percolating water has, 
since then, changed the subjacent limestone to almost pure calcium 
phosphate. At Phosphate Hill, the deposit is in places 10 feet thick, 
while at Murray Hill, a phosphate of aluminum and iron occurs, prob- 
ably derived from the alteration of some voleanic rock. This seems 
to be comparable with Mr. Teall’s description of the phosphatization 
of the trachyte of Clipperton Atoll.!' Besides the massive deposits of 
phosphate, a small quantity occurs almost universally among all the 
limestones of coral islands. The amount has never been found to ex- 
ceed .3 per cent, and is often much less. Its constant character is no 
doubt due to the small quantity which the organisms assimilate from 
the sea-water in building up their skeletons. 
C. Organic residue is only found in the most recent and unaltered 
rocks, and is then present in amount up to 1.5 percent. During the 
processes of consolidation and recrystallization of the rocks, the organic 
matter appears to be dissipated, probably to a great extent in the form 
of carbon-dioxide, and carried off by percolating water. 
Insoluble inorganic residue is present in almost inappreciable amount 
in the very large majority of the rocks. As a rule, the insoluble residue 
varies from .01 to .20 per cent. 
In a few of the limestones immediately associated with volcanic rocks 
such as occur in Christmas Island, Mango, and Guam, the amount of 
residue is larger, and in one case exceeds 4 per cent. This increase 
of insoluble matter is doubtless due to the proximity of the volcanic 
material. With the exceptions cited, the almost entire absence of inor- 
ganic residue is a marked feature of these oceanic coral islands. These 
facts seem to be significant, especially when it is considered that in deep- 
sea deposits, even in the neighborhood of coral islands, the amount of 
insoluble residue rarely falls below 1 per cent, and often rises to 20 
per cent.? 
In the case of the Singatoka cliffs, where the insoluble residue rises to 
nearly 2.5 per cent, the large amount is probably due to the formation of 
the limestone on the margin of the large island of Viti Levu, consisting 
almost entirely of igneous rocks. 
1Q. J. G.S., 1898, pp. 230-232. 
2 Report of Scientific Results of the Challenger Expedition, Appendix III., pp. 
445-496. 
