288 TH 



ists, and various theories have been proposed to explain the ob- 

 served phenomena. We have devoted some little attention to- 

 this part of our subject, and are best satisfied with the explanation 

 given by Mr. Chas. Darwin, in a paper read before the Geologi- 

 cal Society in May 1837 V and which we will explain presently. 

 Everywhere, in the Pacific and Indian Oceans, within tha tropics, 

 may be seen coral banks in their various stages of progress ; some 

 covered with light soil, and the habitations of man. Most of the 

 reefs which raise themselves above the waters are of a circular 

 form, enclosing a basin of still water, called a lagoon r which 

 connects by means of one o-r two channels with the sea. In the 

 interior of the island, the more delicate and smaller kinds of 

 zoophytes live, while the stronger and hardier species, fitted to? 

 endure the beating of the surf, flourish on the outer margin of 

 the isle. When the reef rises so high that it is left uncovered at 

 low water, the corals cease to increase, the animals die, and the 

 branches become somewhat decomposed. Fragments of coral 

 limestone are thrown up by Ihe waves, with shells, and broken 

 fragments of crustacean animals, seeds are floated by the waves 

 towards the new formed island, and thrown upon its shores ; and 

 trunks of trees, drifted thousands of miles, find a lodgment upon 

 it, bringing with them small animals, as insects and lizards. 

 Bushes and trees, spring up, and the sea-birds nestle there, and 

 finally at a later period, it becomes the habitation of man. The 

 reefs of coral, consist not only of the corals, and their broke 

 fragments, but masses of com] act limestone, and imbedded shells 

 are of frequent occurrence. The limestone is found sometimes 

 in the uppermost or newest parts of the reef, and is formed by 

 chemical decomposition, the carbonate of lime being supplied 

 from the decomposition of corals and testacea. 



We have already alluded to the geographical distribution of 

 corals, we may however, form some idea of the immense extent 

 of the coral reefs when we learn that, off the coast of Malabar, 

 in the Indian Ocean, there is a chain of coral islands of over 480 

 miles in length, called the Maldiva Group. On the coast of New 

 Holland, is an unbroken reef 350 miles in length, and between 

 that and the island of New Guinea is a coral formation which 



