354 CORALS AND CORAL L8LANDS. 



is possible that such barriers may often have existed in ancient 

 time, and have disappeared through subsequent denudation of 

 the surface But may not the difference between the great even 

 layers of the continental formations and those of a coral island 

 have proceeded from the difference in the depth of the seas ? 

 Over the great shallow continental seas where the limestones 

 were in progress, the waves may have generally been feeble, 

 and therefore there may have been a less tendency to form nar- 

 row barriers and deep intervening channels. 



The marsh condition of a drying-up lagoon with its forming 

 limestones has been compared above with that under which an- 

 cient unfossiliferous limestones were made. The narrow limits 

 of the former make the comparison unsatisfactory ; for, in the 

 coral island, coarsely fossiliferous beds are all the while form- 

 ing about the exterior of the island, but a few miles at the 

 most from the lagoon-marsh ; while the ancient limestones re- 

 tain their unfossiliferous character often through many thou- 

 sands of square miles. Still, the above mentioned difference be- 

 tween the continental sea and the existing deep oceans may 

 perhaps account for the diversity of results. 



VIII. CONSOLIDATION OF CORAL ROCKS. 



All true coral-reef rocks are examples of the consolidation 

 of material mainly of coral origin — either mud, sands, frag- 

 ments, or standing corals, the last with mud or sands inter- 

 mixed — by (1), an under- water process; (2), at the ordinary 

 temperature ; and they exemplify the mode in which all other 

 submarine limestones of organic origin have been consolidated. 

 The process appears to depend on the presence (proved by 

 chemical analysis) of carbonic acid in the sea waters that bathe 

 and penetrate the sands. This carbonic acid is derived from 

 three sources: from (1), the rains which wash it down from 



