62 THE BERMUDA ISLANDS. 



regions above indicated, and the movements were with little 

 doubt long sustained, and certainly affected large areas at a 

 time. There is nothing, as far as I can see, to indicate that 

 these movements were confined to what is now dry land ; the 

 more natural conclusion is that the axial or plateau uplift ex- 

 tended much beyond the limits of the present peninsula, and 

 as well southward as westward or eastward. The similarity in 

 the geological structure of Yucatan, as it appears from our pres- 

 ent knowledge, lends weight to the supposition that the area 

 thus affected by movements was perhaps continuous completely 

 across the Gulf. 



In explanation of the distinctive form of atolls the ring 

 of coral with its inclosed lagoon it is claimed by the oppo- 

 nents of the subsidence theory that coral plantations building 

 up from submarine banks will grow more rapidly on their 

 outer margins, where the food supply is the greatest, and 

 where, as compared with the inner parts of the mass, there is 

 less obstructive sediment, and thus an exterior rim or eleva- 

 tion would be formed. The differentiation of the inner and 

 outer parts, it is assumed, would be further intensified by the 

 removal in solution of the lime-carbonate from the less active 

 interior portion the region of coral decay and detrital accum- 

 ulation and the formation there of a shallow pan of water or 

 lagoon. That the distinctive features of an atoll may be brought 

 about somewhat in the manner here described can scarcely 

 be doubted; indeed, the supplemental atolls of diminutive size 

 that so frequently accompany the larger reefs, the serpula-reefs 

 of the Bermudas for example, convincingly prove the possi- 

 bility of ring structure without subsidence. But in instances 

 of this kind the ring is^ merely a narrow projection, barely 

 rising above the shallow central depression, and is due prob- 

 ably more to the action of a beating surf than to any other 

 cause. In the case of a true atoll with a large lagoon the con- 

 ditions are very different, and it seems impossible to explain 

 the central depression, often 20, 30, and 40 fathoms, or even 



