117 



in the Indian Ocean, stretch along a line 1500 miles in length, 

 the Maldivas themselves being 470 miles long by 60 miles in 

 breadth. 



The lowness of the islands of Coral formation, over all 

 this vast area, depends upon the fact that Coral-Polyps are 

 powerless to raise their structures higher than the line of 

 low water-mark. To elevate them higher than this, various 

 agencies are at work, but chiefly those of the winds and waves, 

 the force of which detaches masses from the reefs, and piles 

 these up in shallow water, where the interstices become silted 

 up by Coral sand, fine mud, and other debris, of the reef. The 

 general theory of elevation and subsidence of the sea-bottom 

 within the area of the Corallian sea, the conversion of the irregular 

 surface of the reef into one continuous level, and the alterations 

 which its dead and deeply-submerged portions become exposed 

 to in the lapse of time, — are subjects that belong to the 

 consideration of the philosophic geologist, and which have 

 received the most ample illustration and most satisfactory 

 explanation by Mr. C. Dakwin's admirable researches on this 

 most interesting subject.* 



As monuments of past changes. Coral reefs form the basis of 

 some of the most important inductions that have been made in 

 this branch of dynamical geology, and they open up a field of 

 investigation, which has recently yielded some valuable facts 

 regarding the long duration of the life of species in time, 

 and to which I have given full prominence in the section on 

 Fringing reefs. The palseontologist studies in these modern 

 reefs all the slow processes by which the multiplication of the 

 same specific forms through long lapses of time produce changes 

 in the earth's surface, and he is prepared thereby for the 

 investigation of those ancient reefs which existed in the seas of 

 all past ages. Having ascertained that an island occupied each 

 region where now the calm waters of the lagoon lave the Coral 

 strand, he sees in every Atoll a living monument of land that 



* Geological observations on Coral reefs and volcanic islands must be consulted 

 by all who wish to understand the numerous details connected with this subject. 



