CORAL REEFS, &c. 249 



low as fast as growth goes on above. When the reef 

 at last grows up to the surface of the water, the polyps 

 die ; for they cannot live out of water. The winds 

 and waves do the rest ; they break fragments from the 

 sides of the reef and pile them nearer the centre ; they 

 bring sea-weeds and other floating materials, and cast 

 them over the whole ; plants at length spring up, and 

 in the course of years the island except its broad 

 beaches of coral sand is clothed with verdure, and 

 man, perhaps, comes there and makes his home. These 

 little polyps, then, are increasing the amount of dry 

 land on the surface of the globe ; and in this and 

 in other ways God makes their lives serve great and 

 important ends. 



But a history of the polyps would be unfinished if 

 we should not mention their connection with some of 

 the rocks of the globe, the limestones. It is a very 

 interesting fact that reef corals and limestone, or marble, 

 have essentially the same chemical composition ; and 

 it is well known that some of the coral reefs of the Pa- 

 cific, which have been lifted out of water by volcanic 

 forces, are nearly or quite as solid as ordinary marble. 

 Prom these facts, and many others, geologists believe 

 that a large part of the limestones of the globe are 

 made out of the coral reefs that grew in the old oceans, 

 which long before the creation of man covered the 

 countries where marble is now found. If this be true, 

 many of the rocks which underlie vast countries, the 

 marble temples and palaces of the East, the marble 

 monuments and public buildings of our own country, 

 the mortar upon the walls and ceilings 'of our houses, 

 and the marble tables and mantel-pieces so highly 

 11* 



