100 



DYNAMICAL GEOLOQx 



Some species of corals live at great depths and in high 

 latitudes. 



Description of Pacific Coral Reefs and Islands. 



There are in the Pacific two very distinct kinds of 

 islands viz., volcanic islands and coral islands. The 

 former are high, bold, rocky, and often of considerable 

 size ; the latter low, wave-formed. We will suppose the 

 preexistence of volcanic islands, and proceed to show how 

 coral reefs and islands are formed about them. 



Pacific reefs, then, are of three principal kinds viz., 

 fringing reefs, barrier reefs, and circular reefs or atolls. 



1. Fringing Reefs. These grow along any shore- 



PIG. 52. Perspective view of volcanic island and fringing reef. 



line, but the most common and interesting are those about 

 volcanic islands. Suppose, then, a high volcanic island in 

 the midst of the sea. Around such an island corals will 

 build, limited outward by increasing depth, limited inward 



FIG. 53. Ideal section of volcanic island and fringing reef ; s.p., shore platform ; 

 r/>, coral platform. 



by shore-line and upward by sea-level, thus forming a 

 submarine platform clinging close to the island like a 



