106 DYNAMICAL GEOLOGY. 



formed by other agencies, the corals growing only on the 

 outer margin of the bank, because they find the best condi- 

 tions of growth there. Barriers, he supposes, are fringes 

 separated from the encircled island by dying out of the 

 corals on the landward side and extension on the seaward 

 side of the reef. 



It is probable that atolls and barriers are formed in 

 several ways, but under present lights it is not improb- 

 able that many, if not all, atolls are formed as Darwin 

 supposes. We will assume that every atoll marks the 

 place of a drowned volcanic island. 



Area of Subsidence. The area of the subsiding sea- 

 floor is 6,000 miles long and 3,000 miles wide. It is prob- 

 ably not less than 12,000,000 square miles, or greater than 

 the whole North American Continent. 



Area of Laud Lost. This must not be confounded 

 with the sinking area. The sinking area is the whole 

 sea-floor, over millions of square miles ; the land known 

 to have been lost is only the volcanic islands which once 

 overdotted this area. This is, of course, small in com- 

 parison. Estimated by the circles inclosed by atolls, 

 Dana makes it 50,000 square miles. It is doubtless, how- 

 ever, much more than this. For 1. This estimate takes 

 no account of barriers, but all the area between a barrier 

 and the shore-line is also lost. Now, along the Australian 

 coast, for 1,100 miles, there is a barrier thirty to forty 

 miles distant. This alone would make 33,000 square 

 miles lost for this one barrier. We may with confidence, 

 therefore, double the estimate. But, 2. Atolls themselves, 

 as already shown, are smaller, and closed lagoons and 

 lagoonless islands very much smaller than original volcanic; 

 islands. And, 3. In the middle of the coral region there 

 is a blank area of several million square miles, in which 

 there are no islands of any kind. Many islands probably 

 went down here and left no sign, because they went down 

 too rapidly and the corals were drowned. Putting all 



