R. A. Daly — Problems of the Pacific Islands. 167 



being essentially the more or less pulverized remains of shells 

 and skeletons. Emphasis is now increasingly laid on chemical 

 precipitation. The chief agents so far suggested as the indi- 

 rect causes of the precipitation are two kinds of bacteria. 

 The one kind is that to which the decay of albuminous animal 

 matter is due. In the process these minute organisms generate 

 the powerful alkali, ammonium carbonate, which attacks the 

 calcium salts of sea water and precipitates calcium carbonate. 

 According to this view of limestone formation, the decomposi- 

 tion of the soft parts of animals is more important than is the 

 accumulation of their hard parts. 



Drew and others have preferred to attribute the chemical 

 precipitation to the so-called " de-nitrifying" bacteria. Keller- 

 man is now studying the relative significance of the two kinds 

 of bacteria. 



These oceanographic researches are exceedingly valuable for 

 geology and should be extended from Atlantic waters to 

 Pacific waters. The field naturalists who wrote the Royal 

 Society report on "The Atoll of Funafuti " of the Ellice group 

 have already showed what a wealth of information on the 

 limestone and dolomite problems can be obtained from a 

 Pacific island, intensively studied. 



Coral Islands. 



The numerous coral reefs remain for discussion. These are 

 not wholly composed of corals; many other animal and plant 

 species, especially the lime-secreting algse, are bulky constitu- 

 ents. Nevertheless, the corals are essential in giving each 

 reef that amount of strength which enables it to resist destruc- 

 tion by the surf. For that reason the popular name for the 

 composite structures remains good. 



Thousands of them, called " fringing reefs," are growing 

 immediately at the shores of volcanic and other high islands. 

 (See fig. 34, page 185.) 



Hundreds of others, called " barrier reefs," are growing at 

 the outer edges of broad platforms that stretch out from high 

 central islands or from the Australian continent. An illustra- 

 tion is seen in Mbengha, one of the Fiji group. (See fig. 38, 

 page 185.) 



In Oceania about 200 reefs rim the edges of as many open- 

 ocean platforms, which lack central islands ; these are called 

 " atolls." A small example is seen in the symmetrical North 

 Minerva reef, with a longer diameter of about four miles, 

 with a maximum depth in the lagoon of rather more than 100 

 feet, and, as shown by the soundings, with the usual steep 

 slope outside the reef. (See fig. 23, page 178.) Another 



