1889.] Wa/ks Under the Sea by a Coral Strand. 955 
Then if the mountain sinks out of sight, the same processes con- 
tinuing pari passu, the atol is produced, the last Hnk in the chain 
of development. We begin with the fringing reef; we end with 
the atol, a ring of coral, with a central lagoon just over the mount- 
ain top. This is a brief and partial statement of the theory, 
which Dana substantially corroborates. 
But in recent years much new light has been shed upon this 
fascinating subject, and it now appears as if Darwin's theory of 
coral reefs and islands, simple and plausible as it is, is destined 
to be set aside. It can be certainly said that it is not of general 
application. Agassiz found evidence of elevation in the Florida 
reefs, and later Murray, of the Challenger Expedition, has given 
an entirely new explanation of the Pacific coral reef and atol. 
This expedition, sent out to explore the deep seas, was equipped 
with all the appliances which modern science could command, 
and the importance of its results to the scientific world can hardly 
be overestimated. According to this observer, the principal 
factors in the formation of coral structures are the elevation in 
the deep sea of suitable platforms in which corals may build, 
either by volcanic action or by the deposit of organic sediments ; 
the abrasion and solution of the coral rock itself Of the sub- 
sidence which Darwin's view requires, there is no certain proof, 
while on the contrary in the Pacific and Indian oceans there has 
been in many cases an elevation of land. No trace of a sunken 
mountain peak as the base of a coral island has ever been dis- 
covered. The outer wall next the sea is usually not precipitous 
except for the first few fathoms. It then slopes off gradually to 
great depths. Oceanic islands are usually volcanic, and in mod- 
erately shallow waters there is a constant rain of minute solid 
particles to the bottom. These consist of siliceous and calcar- 
eous shells of the minute organisms with which the surface 
waters of the tropical seas are teeming. Having then a suitable 
base on which the corals may build, then the greater growth of 
the margin of the reef, and the erosion of the dead inner parts, 
will account for all the phenomena. As already said the coral 
island is formed by the accumulation of its own debris. 
