74 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
explanation of the formation of atolls and of barrier reefs by the growth 
of the corals of the present epoch. Nor would the geological evidence of 
the great thickness of similar limestones in past periods be of any 
assistance in the solution of the problem if our explanation of the 
formation of atolls and of barrier reefs upon platforms of submarine 
erosion is correct. 
Certainly the analogy of the association of volcanic rocks and so 
called reefs in the Devonian and Trias with similar association in the 
Pacific has no value, as is suggested by Frech? and Langenbeck. The 
substratum upon which corals may grow depends upon the geological 
structure of the country and its latitude. Such an analogy would 
throw out in great part the reefs of Australia, those of Florida, of the 
Bahamas, of Honduras, and of the northern coast of Brazil, which are 
not in volcanic regions, and where the substratum is either Cretaceous 
or Tertiary. 
The only evidence we have of the great thickness of coral reefs, such 
as is required by the Darwinian theory of the formation of atolls and 
of barrier reefs, is based upon the great thickness of the so called 
elevated reefs observed in the Pacific by Dana, Darwin, and others, and 
upon similar observations in Cuba and other parts of the West Indies, 
and upon the evidence of the great thickness of the reefs of the Dol- 
omite. That the latter are true coral reefs is more than doubtful. 
Those rocks are probably great masses of limestone similar to the huge 
deposits of so called elevated reefs of the West Indies and of the Pacific. 
The evidence obtained by boring, and from recent elevated reefs, shows 
that the modern coral reef attains but a moderate thickness well within 
that of the depth at which reef corals grow. The limestones form 
the basis or substratum upon which the recent reefs have obtained 
a footing. The elevated reefs of Cuba and of the West Indies have 
been shown to be Tertiary coralliferous limestones, and the same is the 
case with the elevated reefs of the Pacific, if we can judge of their 
age by that of the elevated coralliferous limestone reefs resembling 
them observed by me in Fiji. 
The central depression, noted as characteristic of the summit of so 
many islands consisting of elevated coralliferous limestones does not show 
these islands to be elevated atolls as has been supposed. The summit 
basin representing the former lagoon of the island has been formed since 
the elevation of the island by atmospheric agencies. This basin is a 
gigantic banana-hole, as such depressions are called in the Dahamas, 
1 Neues Jahrb, f. Mineral., 1892, Pt. II. p. 173. 
