MATERIALS OF CORAL REEFS 393 



in the Andamanian marginal mediterranean (volume salinity ^t, 

 permille) does not prevent their growth. On the other hand, a 

 surface salinity of 38.8 permille, that of the Red Sea (39 permille 

 volume salinity), is not too high for their development. 



Composition and Structure of the Reef. Our knowledge of 

 the composition and structure of the deeper parts of a modern coral 

 reef has been obtained from examination of the accessible parts of 

 reefs and from a deep boring made on the atoll of Funafuti in the 

 Ellis Island group, for the purpose of ascertaining these facts. 

 Three attempts were made, beginning in 1896, and the depth finally 

 reached was 1,114 feet (185 fathoms). (David-22.) This boring 

 passed only through pure limestone of organic origin to the depth 

 reached, without trace of any other rock. The organic remains 

 found were chiefly corals, calcareous algae and Foraminifera, be- 

 sides other types. A boring made from the deck of a ship into the 

 floor of the lagoon, where the water was 17 fathoms deep, showed 

 a deposit consisting of the calcareous alga HaUmcda opuntia, with 

 an abundant admixture of Foraminifera, and having a thickness of 

 over TOO feet. At a depth of 245 feet, IMadrepora, Porites and 

 Heliopora were encountered, forming a solid mass of coral rock. 

 As these corals are shallow-water types, it is evident that subsi- 

 dence has taken place and that the lagoon of Funafuti has been 

 filled up to a depth of at least 245 feet (nearly 41 fathoms). 



Materials Composing the Reef. 



The most important of the reef corals of modern reefs are the 

 branching Madrepores, Pocilloporas and Porites and the heads of 

 Astraeans and Mseandrinas. Among the hydrozoans Millepora is 

 very abundant, the common species being M. complanata and M. 

 alcicornis. In addition to the corals a number of species of nulli- 

 pores occur on the reefs, and often become as important as, if not 

 more so than, the corals themselves. This is the case with the reefs 

 of Christmas Island, of Fiji and Funafuti, where, besides, a promi- 

 nent part is taken by Foraminifera, Echinodermata, Bryozoa, Pele- 

 cypoda and other organisms. 



Within the lagoon of some atolls corals flourish in abundance. 

 At Diego Garcia in the Chagos group the more delicate branching 

 species of the Madreporaria flourish in considerable numbers, and 

 true reef-building species of Porites, Maeandrina, Pocillopora and 

 others are also found. "JNIany of the species growing in the lagoon 

 at a distance of five miles and upward from its outlet are identical 



