STRUCTURE OF CORAL REEFS. 139 



consolidated by a calcareous cement ; and the great abundance 

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

 inated 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 

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

 exuvia of crabs, spines and fragments of Echini, Orbitolites 

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

 inhabiting reef-grounds. 



III. FORMATIONS IN THE SEA OUTSIDE OP 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 100 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 126, figures 

 and describes one of these " coral heads " standing in water fif- 

 ty feet deep, near Turks Island. Its trunk, which made up 



