76 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
off the south coast of Cuba we have along the sides of Bartlett Deep 
fully as steep a pitch as that of any coral reef. 
Gardiner’ has noticed the atoll-shaped shoals inside the great barrier 
reef of Fiji (Plate 23*), and compares them to the smaller atolls inside 
the large atolls of the Maldive group. See also the Serpuline atolls of 
. the Bermudas,’ which hold the same relation to the lagoon inside the 
outer barrier as the atoll-shaped shoals just mentioned hold in Fiji, 
and they certainly are not due to the effects of the débris of the outer 
reefs of the present day, as is claimed by Dana. 
As long as it was taken for granted that the coral reefs of the present 
day were of great thickness, it was natural to compare them with the 
great beds of coralliferons limestones occurring in past ages from the 
time of the Devonian to the end of the Tertiary period. While there is 
abundant proof that some of these limestone beds have been formed 
during a period of subsidence, there is also ample proof that they must 
have greatly increased in thickness by accretion. But it does not follow 
from this that atolls and the lagoons of barrier reefs have been formed 
during a period of subsidence, now that we know that a great many of 
the coral reefs of the present day attain a comparatively moderate thick- 
ness, well within the bathymetrical limits at which reef building corals 
thrive. Nor is there anything to prove that these ancient limestones 
represent such modern structures as the atolls, or bordered the lagoons 
of barrier reefs, and even if they did it is more natural to suppose that 
the lagoons of these ancient atolls, as well as those within the ancient 
barrier reefs, must have been formed by the same agencies which have 
shaped the atolls and the lagoons of barrier reefs in our days. So that 
the analogy between ancient and modern reefs, such as is upheld by Dr. 
R. von Lendenfeld,®? while undoubtedly true as far as it applies to the 
formation of beds of coralliferous limestones of great thickness, yet has 
no application to the actual formation of the lagoon of an atoll or of a 
barrier reef. 
This still leaves open the question of the mode of formation of such 
thick masses of coralline and coralliferous limestone, which, though they 
may originally have been formed partly by subsidence, may also have 
been formed partly by the gradual pushing out to seaward of the outer 
edge of a reef increasing both in height (depth) and in width by the con- 
stant pushing out of the mass of débris and of blocks detached from the 
Loe. cit., p. 498. 
Bull. Mus. Comp. Zool., Vol. XX VI. No. 2, 1895, p. 253. 
Nature, No. 1071, May 8, 1890, p. 29. 
as woe = 
