AGE OF CORAL 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 débris, 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 
little 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 
