no CORALS A XI) CORAL ISLANDS. 



as impalpable and homogeneous in texture as the old limestones 

 of our .continents. There are also other regions where the 

 corals in the rock retain the original position of growth. But 

 the rock in general consists of the debris oi the coral fields, 

 consolidated by a calcareous cement ; and the great abundance 

 of the finer variety of rock indicates that much of it has ori- 

 ginated from coral sand or mud. Wherever broken, it usually 

 presents the character -here described, a texture indicating a 

 detrital or conglomeritic origin. Such a reef-rock is formed 

 in the midst of the waves ; and to this fact it owes many of its 

 peculiarities. Reef-rocks made of corals in the position of 

 growth are formed about the outer reefs wherever the corals 

 grow undisturbed. 



Besides corals, the shells of the seas contribute to it, and it 

 sometmies contains them as fossils, along with bones of fishes, 

 exuvia of crabs, spines and fragments of echini, orbitolites 

 (disc-shaped foraminifers), and other remains of organic life 

 inhabiting reef-grounds. 



III. FORMATIONS IN THE SEA OUTSIDE OF THE BARRIER REEFS. 



While barrier reefs are mostly made up of coarse coral ma- 

 terial, owing to the rough action of the waves, the region im- 

 mediately outside of the breakers, where of much width, is, to a 

 depth of loo feet, one of growing patches of coral and extended 

 surfaces of coral sands. 



Isolated islets of reef-rock are not however of common oc- 

 currence in the middle Pacific, though occurring in large groups 

 like the Feejees. They are most likely to occur where there 

 are great regions of shallow water extending outward from the 

 barrier, and where the tides are not heavy or there is partial pro- 

 tection from them. In some seas, such isolated patches are 

 shaped somewhat like a great mushroom — having'a narrow trunk 

 or column below, supporting a broad shelf of reef above. Mr. 

 J. A. Whipple, in his Journal, referred to on page 99, figures 

 and describes one of these " coral heads " standmg in water 

 fifty feet deep, near Turk's Island. Its trunk, which made up 



