162 FORMATION OF CORAL REEFS. 
contributes to its well-being. Now Corals possess, 
in an extraordinary degree, the power of assimi- 
lating to themselves the lime contained in the salt 
water around them; and as soon as our little 
Coral is established on a firm foundation, a lime 
deposit begins to form in all the walls of its body, 
so that its base, its partitions, and its outer wall, 
which in the Sea-Anemone remain always soft, 
become perfectly solid in the Polyp Coral, and 
form a frame as hard as bone. 
It may naturally be asked where the lime 
comes from in the sea which the Corals absorb in 
such quantities. As far as the living Corals are 
concerned, the answer is easy, for an immense 
deal of lime is brought down to the ocean by 
rivers that wear away the lime deposits through 
which they pass. The Mississippi, whose course 
lies through extensive lime regions, brings down 
yearly lime enough to supply all the animals liv- 
ing in the Gulf of Mexico. But behind this lies 
a question not so easily settled, as to the origin of 
the extensive deposits of limestone found at the 
very beginning of life upon earth. This problem 
brings us to the threshold of astronomy, for the 
base of limestone is metallic in character, sus- 
ceptible therefore of fusion, and may have formed 
a part of the materials of our earth, even in an 
incandescent state, when the worlds were forming. 
But though this investigation as to the origin of 
