212 



MARINE ANIMALS 



meters above sea level, and often more or less interrupted by pas- 

 sages (Fig. 41). The largest atoll, Sunadiva, is 42 by 32 miles with 

 a perimeter of 121 miles and contains over a hundred subsidiary islets 

 and a lagoon up to 100 m. deep which connects with the open sea by 

 40 channels. 



Darwin 63 explained these varied reef formations with a comprehen- 

 sive theory based on the following considerations: (1) that reef corals 



Fig. 41. — The atoll of Pinaki, Tuamotu Islands. After Agassiz. 



flourish only to a depth of 40-60 m.; (2) that the growth is most rapid 

 on the outer face of the reef; and (3) that variations in the level of 

 the sea bottom take place. From the first consideration, reefs cannot 

 rise to the surface from the oceanic depths, but must be confined 

 to submarine banks, such as the tops of submerged volcanos, or on the 

 coast of existing land. On stationary and on rising coasts, the reefs 

 are consequently of the fringing type (Fig. 42, I). But when mainland 

 coast or islands are sinking, the outer, more rapidly growing edge of 



Fig. 42. — Origin of various forms of coral reefs according to Darwin : I, fringing 

 reef; II, barrier reef; III, atoll. Cross sections above, outline below, in each 

 case. 1, 2, sea level in I and II respectively. 



the reef will be separated more and more from the shore, and a channel 

 or lagoon without corals is consequently produced between the reef 

 and the original coast, i.e., a barrier reef is formed (Fig. 42, II) . When 

 the submergence is long-continued, the island itself may disappear, 

 and only the reef, now an atoll, remains (Fig. 42, III). "Atolls are the 

 grave stones of sunken islands." All intergradations between barrier 

 reefs surrounding islands and atolls may be found. Volcanic islands 

 with surrounding reefs are found in the Gambier archipelago. In the 

 Society Islands the progressive disappearance of the volcanic peaks 



