CORALS. 129 



As the amount of sand increases it collects in such quantities as to be a detriment 

 to the growth of the coral animals themselves. It covers not only the foundation of 

 the coral island but extends far and wide over the coral plantation upon which the 

 island rests, and tends to kill the great colonies of life by which this platform is peo- 

 pled. The part of the reef where the coral life still lives now retreats into the ocean 

 to the greatest possible distance from the sand. 



The winds, tides, and rains continue the work which they have taken up. Ocean 

 currents, especially, perform a most important part in the many changes which take 

 place. The problem now becomes a geological one, and wholly independent of those 

 ■of animal life. 



The continual wear and tear resulting from the erosion of the water on the coral 

 island in its second stage of growth is ever increasing the amount of broken fragments 

 near the low-water line, while the winds snatch the sand thrown up by the waves and 

 heap it into high banks, dipping invariably to the sea. Exposure to the air and other 

 ■causes now exert their influences upon it, and the coral sand is hardened into a soft 

 rock, the well-known coral rock of these islands. The island is now foi-med? 

 Not yet. To the stability of the precarious foundation thus made few persons could 

 with impunity trust themselves through a tropical hurricane. There are other changes 

 before the coral island becomes firm, and its career has just begun. 



In the progress of time the processes of erosion go on, and the waves and rains eat 

 their way into the soft rock so that the island is honeycombed by the erosion. Once 

 more the rock is reduced to fine sand, and scattered far and wide over the submarine 

 flats. The softer, least consolidated layers of the rock, which, from not being exposed 

 to the air, are below the harder, wear away faster than the upper strata, which are 

 thus broken off in large fragments. The products of the erosion are strewn far and 

 wide over the coral platform, and heaped up into a new island, of a different form 

 from that which existed before. One of two things now takes place with the debris. 

 Either the sand is again thrown up on the forming island, to again harden into coral 

 rock, or it is swept over neighboring growing coral reefs, raising these one more stage 

 to the level of the surface of the ocean. The movement by the wind of this sand 

 upon a coral island of considerable size often assumes formidable proportions. The 

 constant winds on some coral islands, as the Bermudas, heap the coral sand into dunes 

 ■of considerable elevation, which slowly move en masse, engulfing everything which 

 lies in its path. One of the most interesting of these moving masses of sand is to be 

 ■seen in Paget Parish, in Bermuda. The sand on the south shore of this parish has in 

 its motion suggested the name of a " sand glacier," and for several years it was slowly 

 making its way inland from the coast, covering to a considerable depth farms, and even 

 a farmhouse, in its course. Artificial means of staying its progress had to be resorted 

 to, and by planting trees in the line of its onward motion the progress of the sand was 

 stopped. 



In sheltered coral lagoons, whose floor is formed of coral sand aj)2>roaching the sur- 

 face of the water, or with but a moderate depth, the mangrove trees oftentimes furnish 

 & nucleus around which characteristic coral islands, known in Florida as mangrove keys, 

 are formed. A small mangrove, sending its root into the submerged sand, forms an 

 obstruction upon which catch floating seaweeds and similar organisms. As the man- 

 grove increases in size, and its spreading branches send down rootlets which fasten 

 themselves more firmly in the sand below, the obstruction is increased in size, and the 

 island grows with every increment to its size, until eventually we see a coral island of 



VOL. I. — 9 



