104 DYNAMICAL GEOLOGY. 



lagoon open to the sea on one side (Fig. '58, ), or the 

 lagoon may be entirely closed (Fig. 58, c), or the ring may 

 close in upon itself so as to abolish the lagoon (Fig. 58, d). 

 These are so different from the typical atoll that they 

 may be considered a fourth class. 



Theory of Barriers and Atolls. 



Fringing reefs need no theory. Corals, finding the con- 

 dition of suitable depth along the shore, build upward 

 to the sea-level and outward to the depth of one hundred 

 feet, and thus form a coral platform clinging to the orig- 

 inal island. But barriers seem at first sight to form far 

 from land in abyssal depth ; and atolls seem to form in 

 deep sea without any island-nucleus. These facts seem to 

 violate the conditions of coral growth. How are they 

 explained ? The most probable explanation was first 

 given by Mr. Darwin. 



Darwin's Subsidence Theory. According to Dar- 

 win, every reef began as a fringe, and would have remained 

 so if the floor of the ocean had remained steady. But, in 

 all the region of barriers and atolls, the ocean-floor has 

 slowly subsided, carrying all the volcanic islands with it 

 downward. Now, if the subsidence had been more rapid 

 than the coral ground could rise by accumulations of debris 

 of successive generations, then the corals would have been 

 carried below the depth of one hundred feet and drowned. 

 But the subsidence was not faster than the coral ground 

 could be built up. Therefore the corals building upward, 

 as it were, for their lives, kept their heads at or near the 

 surface. But the reef, building up nearly at the same 

 place, while the volcanic island grew smaller, it is evident 

 that the latter would be separated more and more from 

 the reef. When the island was down waist-deep, the reef 

 was a barrier ; when down head-under, it became an atoll, 

 the reef representing nearly the outline of the original 

 base of the volcanic island. We said nearly, but not per- 



