AGE OF COKAL REEFS. 179 



materials is formed about a Coral Reef. Tides 

 and storms constantly throw them up on its sur- 

 face, and at last a soil collects on the top of the 

 Reef, wherever it has reached the surface of the 

 water, formed chiefly of its own debris, of Coral 

 sand, Coral fragments, even large masses of Coral 

 rock, mingled with the remains of the animals 

 that have had their home about the Reef, with 

 sea-weeds, with mud from the neighboring land, 

 and with the thousand loose substances always 

 floating about in the vicinity of a coast, and 

 thrown upon the rocks or shore with every wave 

 that breaks against them. Add to this the pres- 

 ence of a lime-cement in the water, resulting 

 from the decomposition of some of these mate- 

 rials, and we have all that is needed to make a 

 very compact deposit and fertile soil, on which a 

 vegetation may spring up, whenever seeds float- 

 ing from the shore, or dropped by birds in their 

 flight, take root on the newly formed island. 



There is one plant belonging to tropical or sub- 

 tropical climates that is peculiarly adapted by its 

 mode of growth to the soil of these islands, and 

 contributes greatly to their increase. This is the 

 Mangrove-tree. Its seeds germinate in the calyx 

 of the flower, and, before they drop, grow to be 

 Ifttle brown stems, some six or seven inches long, 

 and about as thick as a finger, with little rootlets 

 at one end. Such Mangrove-seedlings, looking 



