CORAL ISLANDS AND REEFS 85 



heights, 1,000 feet or more, and are surrounded by reefs 

 of a later formation. Such islands may be isolated, 

 rising out of depths of 1,000 or 2,000 fathoms. Examples 

 of these are Christmas Island in the Indian Ocean, 

 Metia and some of the Pelews in the Pacific, and 

 Bermuda in the Atlantic. In Fiji high elevated coral 

 islands are abundant, associated with volcanic islands 

 and with landless coral reefs ; every such island seems 

 to have been formed by a separate and apparently 

 rapid upheaval, not by a movement of elevation which 

 affected the whole group. Many other islands, such 

 as are ordinarily called " coral islands," are quite low, 

 rising only a few feet above the sea-level. They rest 

 upon the surface of broad reefs, coral plateaux at the 

 surface of the sea ; and the method of formation of 

 these coral reefs is the important question. To avoid 

 misconception we would at once point out that there 

 is no adequate proof that any single coral reef owes 

 its existence to the gradual sinking of the foundations 

 upon which it has been built, while there is plenty of 

 evidence that coral reefs can arise by other means, 

 and grow into the regular ring-shape atolls so char- 

 acteristic of the Indo-Paciflc Ocean. 



Coral reefs are found between lat. 30 N. and lat. 

 30 S., where the surface temperature of the sea does 

 not fall below a minimum of about 70 F. Within 

 these limits they exist off nearly every land in the 

 Indo-Pacific Ocean, and form a vast number of small 

 isolated rings, most of which are crowned with land, 

 and garlanded with coconut palms. In the Atlantic 

 they are found in the West Indies, off Florida, and in 

 places off the coast of Brazil, but they do not form 

 isolated peaks in the ocean bed or exist around 

 its other islands, except Bermuda. Further, on the 

 eastern shores of all oceans typical coral reefs do not 



