34 Mr Maclaren on Coral Islands and Reefs, as 



and at a greater depth the Madrepore and Astrea are beheved 

 to exist. On the exterior margin of the reef at the surface, 

 the Porites were in irregularly rounded masses from four to 

 eight feet broad, nearly of equal thickness, and divided from 

 each other by narrow crooked channels about six feet deep. 

 Other parts of the reef were composed of thick vertical plates 

 {JSliUepora co?«j»/aHa^a), intersecting each other atvarious angles, 

 and "forming an exceedingly strong honeycombed mass.'' 

 Between these plates and in protected crevices, a multitude of 

 branching corals live, and the lagoon is inhabited by^a distinct 

 set of corals, generally brittle and thinly branched. The 

 Nulliporse, which have no visible cells, and though resembling 

 corals, are supposed to be plants, occasionally cover the 

 Porites and Millipores up to the level of high water. 



Coral Reefs and Atolls. — These reefs are submarine rocks of 

 coral, usually ascending so near to the surface of the sea that 

 their existence is indicated to the navigator by breakers. They 

 are found remote from land, are in vast nvmibers, and often of 

 great extent, and generally affect an irregularly circular form, 

 having a pool of comparatively still water in the middle, called 

 a lagoon. Storms throw up masses of broken coral upon them, 

 which accumulate to the depth of some feet above high-water, 

 forming chains of islets along the reef. The whole reef in this 

 condition is called a " lagoon island," or more conveniently an 

 " atoll," a word borrowed from the South Sea islanders. Some 

 reefs have many islands upon them, some have few, and some 

 have none. 



A coral reef may be defined a wall or mound of coral rock, 

 built up in the ocean from a considerable depth, and generally 

 returning into itself, so as to form a ring, with a sheet of still 

 water in the interior. " Every one," says Mr Darwin, " must 

 be struck with astonishment when he first beholds one of these 

 vast rings of coral rock, often many leagues in diameter, here 

 and there surmounted by a low verdant island with dazzling 

 white shores, bathed on the outside by the foaming breakers 

 of the ocean, and on the inside surrounding a calm expanse of 

 water, which, from reflection, is of a bright but pale green 

 colour."' The wall of coral rock forming the ring, is generally 

 from a furlong to half a mile in breadth, averaging about a 

 quarter of a mile. In one rare case it is three miles. The 



