150 ORGANIC AGENCIES. 



(Fig. 124, c). Sometimes the lagoon closes up, and a lagoonless island 

 is the result (Fig. 124, d). These different forms graduate into one 

 another and into the typical atoll. 



Theories of Barrier caul Circular Reefs. 



Fringing reefs require no theory. Corals attach themselves to the 

 shore-line because they find there the depth necessary for their growth, 

 and they extend outward until they are limited by the increasing depth. 

 But there is a real difficulty in explaining barriers, for they seem to rise 

 from water too deep for coral-growth ; and the difficulty becomes still 

 greater in the case of circular reefs or atolls, for these seem to have no 

 connection with any pre-existing land, but to grow up from an un- 

 fathomable bottom. These latter, by their singularity and extreme 

 beauty, have always attracted the attention and excited the wonder of 

 Pacific travelers ; and to their explanation theories have been princi- 

 pally addressed. 



Crater Theory. — This theory supposes that an atoll is an extinct 

 submarine volcano, the lagoon being the crater and the reef the lip or 

 margin of the crater ; that corals finding on this circular rim the con- 

 ditions of depth necessary for their growth, occupy and build upon it to 

 the surface of the water, after which, of course, waves finish the work 

 by beating up the islets. The incredible supposition that thousands of 

 these volcanoes should have come within 100 feet of the surface, and 

 yet none of them appear above the surface, is not necessary ; for we 

 may suppose that many of them were originally above the surface, 

 but, being composed of ashes and cinders, have been washed down by 

 the waves. In 1831 a volcano burst forth in the Mediterranean and 

 quickly formed an island of cinders and ashes, called Graham's Island. 

 In a few months this island was entirely washed away by the waves, 

 and only a circular submarine bank remained. If corals grew in the 

 Mediterranean, there seems no reason why a circular reef should not 

 have been formed. 



Objections. — Even in its most plausible form, however, this theory 

 is very improbable as a general explanation of atolls : 1. The great 

 size of some of these atolls — thirty, sixty, and even ninety miles in 

 diameter ; and, 2. The high angle of the slope of these submarine 

 mountains — 50° to 60° or more — seem inconsistent with their volcanic 

 origin. 3. This theory offers no explanation of the barrier reefs, and 

 yet it is possible to trace every stage of gradation between barriers and 

 atolls, showing that they are due to similar causes. 



Subsidence Theory of Darwin. — This theory explains not only 

 atolls, but also barriers, and connects both in a satisfactory manner 

 with fringing reefs. It supposes that the sea-bottom, where atolls and 

 barriers occur, has been for ages subsiding, but at a rate not greater 



