2o8 SEA AND LAND 



almost all cases communicates with the open sea by one or 

 more channels. If the waters of the Pacific and Indian 

 Oceans in the districts occupied by these atolls could be 

 drained away, the observer would bften be able to journey 

 for many hundred miles through fields occupied by towering 

 mountain-like elevations, each rising with steep and regular 

 slopes to a certain level, and nearly all of them containing 

 a shallow, cup-shaped cavity on the summits. In some cases 

 these mountains would be in figure very nearly true cones, 

 but by far the larger number of them have elongated forms. 

 Here and there in the Pacific Ocean where the sea-floor has 

 been subject to recent elevation, these atolls have been lifted 

 until their summits are some hundreds of feet above the 

 plane of the sea. So far as has been observed, these high- 

 lying coral islands have lost their characteristic cup-shaped 

 summits by the process of erosion. 



There is much debate at present as to the origin of these 

 interesting atolls. Until recently the explanation adduced 

 by Charles Darwin found universal acceptance. This was 

 in effect that the coral reefs began to grow in the form of 

 a barrier surrounding some island of ordinary rock such as 

 is often formed in the sea by a partially submerged moun- 

 tain peak or a volcanic cone. The sea-floor on which the 

 elevation rested being in process of gradual subsidence, but 

 at a rate not sufiicient to destroy the corals of the reefs, the 

 living rock gradually built upward until when the original 

 land disappeared beneath the waters nothing was left to mark 

 its former site except the limestone deposits formed by the 

 zoophytes and ()th(.;r animals which continued to develop upon 

 its submerged summit. One of the proofs of this hypothesis 

 was found in the occasional cases where ordinary islands 



