- ~ a" roe _—* = 2 
\ $e ee Pi ph ei ee 
there considerable beds of broken shells have been produced by the ~ 
accumulation of the excrement of fishes, as Verrill has poimted out 
on the north-eastern coasts of the United States. | 
Coral-reefs.'.—But the most striking calcareous formations now 
in progress are the reefs and islands of coral. These vast masses of 
rock are formed by the continuous growth of various genera and 
species of corals, in tracts where the mean temperature is not lower 
than 68° Fahr. Coral-growth is prevented by colder water, and is 
likewise checked by the fresh and muddy water discharged into the 
sea by large rivers. Hence many coast-lines in tropical seas are 
destitute of coral-reefs. 
Darwin and Dana have shown that reef-building corals cannot 
live at depths of more than about fifteen or twenty fathoms. When 
they begin to grow, either fronting a coast-line or on a submarine 
bank, coral reefs continue to advance outward, the living portion 
being at the surface, while the mass underneath consists of a cal- 
eareous skeleton which becomes a solid white compact limestone. 
In the coral area of the Pacific there are, according to Dana, 290 
coral islands, besides extensive reefs round other islands. ‘The 
Indian Ocean contains some groups of large coral islands. Reefs of 
coral occur less abundantly in the tropical parts of the Atlantic, among 
the West Indian Islands and on the Florida coast. The great reef 
of Australia is 1250 miles long and from 10 to 90 miles broad. 
Coral rock, though formed by the continuous growth of the 
polyps, gradually loses any distinct organic structure, and acquires 
an internal crystalline character like an ancient limestone, owing to 
the infiltration of water through its mass, whereby calcium carbonate 
is carried down and deposited in the pores and crevices as in a growing 
stalactite. Great quantities of calcareous sand and mud are produced 
by the breakers which beat upon the outer edge of the reefs. This 
detritus is partly washed up upon the reefs, where, being cemented 
by solution and redeposit, it aids in their consolidation, sometimes ac- 
quiring an oolitic structure,? but in great measure it is swept away 
by the ocean currents and distributed over many thousands of square 
miles of the sea-floor. 
As already mentioned (p. 282), the formation of coral islands has 
been explained by Mr. Darwin on the hypothesis of a subsidence of 
the sea-floor. These circular coral islands, or atolls, rising in mid- 
ocean, have the general aspect shown in Fig. 174. ‘Their external form 
may be understood from the chart (Fig. 175), and their structure and 
the character of their surface from the section (Fig. 176). They 
rise with sometimes tolerably steep slopes from a depth of 2000 feet 
and upwards, until they reach the surface of the sea. But as the 
coral polyps do not live at a greater depth than about 15 or 20 fathoms, 
1 See Darwin, “The Structure and Distribution of Coral Islands,” 1842; Dana, 
“ Oorals and Coral Islands,” 1872; Jukes’ “ Narrative of Voyage of H.M.S. Fly,” 1847 ; 
Murray, Proc. Roy. Soc. Edin. x. p. 505. 
2 See Dana’s “ Corals and Coral Islands,” pp. 152, 194. 

464 _ DYNAMICAL GEOLOGY. [Book IIT. _ 
’ 

