263 THE POPULAR SCIENCE MONTHLY. 



states that the astroeas occur at the bottom of the reef. Next in order, 

 the brain-coral, pontes, madrepores, and on the surface the light, 

 branching varieties which are the shrubbery of the coral world. This 

 arrangement may occur where the bottom on which the reef stands is 

 immovable, or has remained without change of level since the reef 

 commenced. But how it could occur in a coral mass 2,000 feet thick, 

 growth being limited to a depth of 120 feet, is not entirely obvious. In 

 order to explain the enormous depth of coral-reefs upon the submerged 

 lands of the Pacific, it is necessary to consider the further well-estab- 

 lished fact, first suggested by Charles Darwin, that the lands and ocean- 

 bed have gradually subsided. The subsidence has been often at the same 

 slow rate that coral-reefs have increased by upward growth. It seems 

 inevitable from this that the builders are at w T ork on the upper portions 

 of the reef; certainly it is here only that the work of elevation can go 

 on. Summing up this subject, Prof. Dana says : " Reef-building corals 

 of the different groups grow together promiscuously at different depths 

 up to low-tide level. The largest astrseas, mseandrines (brain-coral), 

 porites, and other kinds, have been seen by the author, constituting the 

 upper part of the growing reef." The coral polype flourishes only in 

 the belt of warm waters which lies in and near the tropics. A tem- 

 perature lower than 68 Fahrenheit is fatal to them. The great reefs 

 abound and grow w r ith greatest vigor in the zone of greatest heat. 



Surrounding most of the tropical islands are two principal reefs, one 

 fringing the shore ; the other, called the barrier-reef, lying seaward, 

 sometimes more than 15 miles from the land. The intervening space 

 is often filled with minor reefs and a gorgeous wealth of coral vegeta- 

 tion. 



Here lie immense platform-reefs, a shell of coral covering the bottom- 

 beneath the shallow waters. These together make up the coral-reef 

 ground of the island. West of the two larger Feejee Islands are 3,000 

 square miles of reef-ground. New Caledonia has a reef along its western 

 shores a distance of 250 miles. The great Australian barrier, lying east 

 and northeast of that island, forms a broken reef, 250 miles in length. On 

 these outer reefs the waves forever break, and here, where the plunge of 

 the surf is most furious and persistent, the polypes flourish with great- 

 est vigor, and open their many-colored petals to the life-giving waters, 

 as do thirsty flowers to the welcome rain. On every dead space deli- 

 cate moss-like and lichen-like corals quickly form their thin, hard 

 crusts. 



Outside the reefs there occurs, in many places, a coral growth alike 

 curious and interesting. In isolated patches are found immense mush- 

 room-shaped masses called coral-heads. One is described by Whipple, 

 cited by Dana, standing in water 50 feet deep, near Turk's Island. 

 Its trunk is about 15 feet in diameter, supporting a tabular mass 

 100 feet in diameter, the top being bare at low tide. When these 

 corals reach the surface, growth in that direction ceases, but may 



