988 avs |, CORALS AND CORAL ISLANDS. 
the elevated coral island, Metia, two hundred and fifty feet 
high, the stratification is horizontal; the terraces of Christ- 
mas Island are horizontal; and the same is true of the ele- 
vated reefs of Cuba. 
If a satistactory decision in any case is not arrived at by 
the methods or criteria which have been described, there is 
the best of evidence to be had by artesian borings that shall 
go to the bottom and bring up a core six inches in diameter. 
It may thus be decided whether the limestone passed through 
is of reef-coral, or talus-debris, or partly or wholly of pelagic 
constitution." 
2. Courses of reefs and channels determined by marine 
currents.— The view has been urged by Lieut. E. B. Hunt, 
U.S. N. and Messrs. A. Agassiz, Murray, Semper, and Guppy 
that the positions of reefs off coasts are sometimes and gener- 
ally located by drift currents at considerable distances from 
coasts, and hence that the wide intervals of water inside of 
barrier reefs may result without aid from subsidence; and 
Dr. Guppy has urged, as remarked on another page, that 
atolls may be so shaped. 
a. The facts, presented by Lieutenant Hunt, and more 
fully by Mr. Agassiz with regard to the effects of the eddy 
current of the Gulf Stream have been mentioned on page 207. 
They show that coral reefs may be elongated, and also that 
1 The kind of submarine slopes to be ordinarily looked for off coral reefs is 
ilustrated by the Challenger soundings. And it is interesting to note that the facts 
sustain instead of correcting those announced by earlier observers. Beechey and 
Darwin make the mean slope about 45°, and my Report says 40° to 50°. I have 
assumed for the slope of the bottom outside of the reef-limit the same angle as for 
the surface-slope of the island just above the water-level ; 5° to 8° off Tahiti, of 
which 5° is accepted as most correct, and 3° to 5° off Upolu; and the assump- 
tion as regards Tahiti is sustained by the Challenger soundings. My Report 
states (from the Expedition surveys) that off Upolu, the bottom “loses more and 
more in the proportion of coral sand till we finally reach a bottom of earth,” and 
introduces this as an argument against the indefinite drifting of coral sands into the 
deep ocean; and this argument the Tahiti soundings sustain. 

