SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. 119 
V. Conclusion. 
In reviewing the facts which a chemical and microscopical examination | 
of these coral limestones has elicited, it will only be possible at this stage 
of the inquiry to notice briefly three of the more prominent questions 
which arise. These are: — (A) The constitution and probable origin of 
the coral limestones, (B) their geological age, (C) the relation of the 
distribution of magnesium carbonate in the limestones to the question of 
the origin of dolomite. 
A. The published accounts of the several islands, together with the 
detailed microscopical examination of the rocks, show that the lime- 
stones, apart from the bedded basal limestones dipping at 15°, are 
mainly of two types. True coral reefs occur here and there among the 
older rocks of the different islands, but are usually confined to the 
modern fringing reefs, which are now found on the slopes of the elevated 
islands, and were probably formed during pauses in the movement of 
upheaval. 
The greater part of the masses of these upraised coral islands consists, 
however, of fragmental rocks, made up of pieces of reef-forming corals, 
together with the other organisms usually associated with corals in a 
reef. Globigerina and other deep-sea forms are found occasionally, but 
only in numbers approximating to those met with drifted into the 
lagoons of existing reefs. The evidence of the organisms as a whole, 
together with the intercalation of true reefs, shows that these frag- 
mental deposits were laid down under shallow water conditions close 
to a true reef. The great thickness of these rocks may be accounted 
for in some cases by the outgrowth of a reef on its own talus during a 
period in which no vertical movement took place. Mr. E. C. Andrews 
is of the opinion that the bedded limestones dipping at 15° and inter- 
bedded with “ soapstones,” which form the basal rocks in the Fijis, were 
formed by subsidence, but that the unbedded limestones and reefs of 
later age were formed during intermittent periods of elevation, and 
form only a veneer of coral limestone, more or less perfectly masking 
the older bedded foraminiferal rocks. 
» B. The age of the older central limestones which form the greater 
part of the elevated coral islands has been determined with a fair degree 
of certainty in a few cases. 
Prof. Rupert Jones and Mr. Frederick Chapman,! when examin- 
ing the Foraminifera from Christmas Island, discovered in the oldest 
1 Monograph of Christmas Island, 1900, pp. 226-264. 
