RELATION TO THE SHORE-LAND 207 



through the reef and extend for some distance across the floor 

 of the lagoon. 



While alono;- the coasts of the mainlands and greater islands 

 where coral reefs occur they commonly appear in the general 

 form of broad shelves with the rim of hi^^her and livino- reef 

 next the sea and a prevailing shallow lagoon or channel between 

 this barrier and the shore, their form is sometimes much affected 

 by the upward and downward movement of the shore-land dis- 

 trict against which they lie. An elevation such as frequently 

 occurs and has lately happened along the coast of Florida may 

 bring the reef and the lagoon as well above the level of the 

 sea ; in some cases successive elevations have thus developed 

 a considerable breadth of shore country composed of reef 

 deposits. Through these easily dissolved rocks the river waters 

 may carve, mainly by their dissolving action, tolerably deep 

 channels which may serve as harborages. In other cases the 

 subsidence of the coast may lower the barrier reef more rapidly 

 than the coral animals can effect its elevation, and in this way 

 the ridge which at one time sheltered the coast from the waves 

 may no longer afford such protection. 



The group of atolls or annular reefs is imperfectly repre- 

 sented in the Atlantic district, but attains a remarkable devel- 

 opment in the Pacific and Indian Oceans. In their character- 

 istic forms these wonderful islands consist of a steeply sloping 

 obscurely conical elevation rising from the floor of the sea, 

 which often exceeds a mile in profundity, to the surface of 

 the water. Above the sea level there is a more or less com- 

 plete ridge composed of decayed and broken-up coral which 

 has been swept into position somewhat inside of the living 

 reef by the action of the waves. The central parts of the 

 island are occupied by a shallow basin of water which in 



