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 
erowth 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), the tubes of Serpule or sea-worms, 
and other remains of organic life inhabiting reef-grounds. 
, 11. 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 50 to 150 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 fit- 
ty feet deep, near Turks Island. Its trunk, which made up 
