Hicks or. the fReef=(Builders. 303 
filling the interstices as fine sand and precipitated from solution 
in the sea, would help to form and maintain a steep face. The 
position of the talus just beneath the zone of living corals ac- 
cords with Murray's theory. In fine the facts are against sub- 
division as a necessity in the explanation of thick reefs. 
But while thickness of coral reefs is not in itself a positive 
proof of subsidence, there may be other attendant circumstances 
which do constitute good evidence of it. If, for instance, a reef 
is very thick and at the same tin.e very steep in the lower part 
of its sea-ward face (about the region d in fig. i) we may rea- 
sonably infer subsidence. "Seven miles east of Clermont Ton- 
nere the lead ran out to 1145 fathoms (6870 feet) without 
reaching bottom. Within three quarters of a mile of the south- 
ern point of this island the lead, at another throw, after running 
out for awhile, brought up an instant at 350 fathoms, and then 
dropped off again and descended to 600 fathoms without reach- 
ing bottom. On the lead, which appeared bruised, a small 
piece of white coral was found, and another of red."^ This 
sounding indicates vertical or overhanging walls at great depths. 
Several examples of the same kind are on record, and are good 
evidence of subsidence, although such wall-like steepness is 
quite exceptional. Great thickness of a reef on the inner or 
lagoon side is also good evidence of subsidence, at least until we 
have more complete proof of the adequacy of Murray's solution 
theory. Dr. Archibald Geikie, who in other respects accepts 
Murray's views, doubts the competency of sea water to dissolve 
such vast masses of limestone as must have been removed in 
some cases. Prof. Carl Semper accounts for deep lagoons by 
the scouring action of tides and currents, but Dana replies that 
the waters can scour only to the depth of the outlet from the 
lagoon to the sea, which is sometimes of less depth than the 
lagoon. Deep lagoons, therefore, seem to present the strongest 
case for the Darwinian theory. 
Atolls are placed by Darwin at the end of a series, beginning 
with the fringing reef, then the barrier reef, then the atoll; and 
upon that supposition they must always have been preceded by 
subsidence. Dana accepts this view, and makes it the basis of 
' Dana. Corals and Coral Islands, p. 171. 
