ILLUSTRATIONS (7). CORAL-REEFS. 261 



Silliman, the genus Porites, like many other cellular coral- 

 trunks (Madrepores, Astrseas, and Mseandrinas of Ceylon and 

 the Bermudas), contains besides from 92 to 95 per cent, of 

 carbonate of lime and magnesia, a portion of fluorine and 

 phosphoric acid.* The presence of fluorine in the hard 

 skeleton of the polyps reminds us of the fluoride of calcium 

 found in fish bones according to Morechini's and Gay-Lus- 

 sac's experiments at Rome. Silex is mixed only in very 

 small quantities, with the fluoride of calcium and phosphate of 

 lime found in the coral-trunks; but one coral animal allied 

 to the Horn corals (Gray's Hyalonema, Glass thread) has 

 an axis of fibres of pure silex, resembling a hanging tuft 

 of hair. Professor Forchhammer, who has recently been 

 engaged in a thorough analysis of sea-water in the most op- 

 posite parts of the earth's surface, finds the quantity of lime 

 in the Caribbean Sea remarkably small, it being only T -- !_, 

 whilst in the Cattegat it amounts to T f Ji^-. He is disposed 

 to ascribe this difference to the numerous coral-banks 

 near the West India Islands, which appropriate the lime to 

 themselves, and thus exhaust the sea- water. f 



Charles Darwin has with great ingenuity developed the 

 genetic connection between shore-reefs, island-encircling 

 reefs, and lagoon islands, i. e., narrow, annular coral banks 

 which surround inner lagoons. According to his views, 

 these three kinds of structure depend upon the oscillating 

 condition of the bottom of the sea, or on periodical elevations 

 and subsidences. The often-advanced hypothesis, according 

 to which the lagoon-islands, or atolls, mark by their circularly 

 enclosed coral-reefs, the outline of a submarine crater, raised 

 on a volcanic crater-margin, is opposed by the great extent of 

 their diameters, which are in some instances upwards of 30, 

 40, or even 60 miles. Our fire-emitting mountains have no 

 such craters, and if we would compare the lagoon, with 

 its submerged mural surface and narrow encircling reef, with 

 one of the annular lunar mountains, we must not forget that 

 these annular mountains are not volcanoes, but tracts of land 



* Compare James Dana (geologist in the United States' Exploring 

 Expedition under the command of Captain Wilkes), On the Structure 

 and Classification of Zoophytes, 1846, pp. 124 131. 



f Report of the Sixteenth Meeting of the British Association for the 

 Advancement of Science, held in 1846, p. 91. 



