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 liine 

 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 



