SOME ANIMAL ARCHITECTS. 143 



on the space of earth allotted to them. . . . Being in the 

 middle of an atollon, you see all around you this great stone 

 bank, which surrounds and protects the island from the 

 waves ; but it is a formidable attempt, even for the boldest, 

 to approach the bank and watch the waves as they roll in 

 and break with fury upon the shore." Soundings on the 

 seaward side of an atoll reveal abyssal depths as in the case 

 of barrier reefs ; and, as in the latter instance, the living 

 corals exist only at the surface margins of the reef, extending 

 downwards merely to their natural limit of depth. The 

 massive living corals, as before, flourish best where the surf 

 is of the heaviest description, whilst more delicate kinds 

 grow within the quiet waters of the lagoon ; and it must be 

 borne in mind that the living corals, of whatever variety, 

 require to be constantly immersed in their native waters. 

 Exposure, even for a limited period, to the rays of the sun, 

 is fatal to their vitality. From this observation it becomes 

 clear that the labours of the coral-polypes are inadequate to 

 raise the reefs above the surface of the water ; other agencies, 

 as will be presently noted, completing the erection, by the 

 addition of foreign matter for the elevation of the reef. The 

 depth of the central lagoon varies, a depth of forty-nine fathoms 

 being of maximum kind; and the bottom of this central 

 lake consists usually of sand and soft clay, or fine calcareous 

 mud, the latter resulting from the grinding-down process to 

 which the coral is subjected by fishes and other enemies. 

 The coral ring of the atoll is broken at one or more points 

 by an opening, often large enough to admit the passage of 

 large ships into the quiet haven within. This passage into 

 the lagoon invariably occurs on the leeward aspect of the 

 atoll ; this fact possessing a practical interest for the navi- 

 gator who has succeeded in safely avoiding the dangerous 

 swell and surf of the seaward side. 



Noting these three varieties of coral-reefs, we may now 

 proceed to inquire into the chief theories which from time 

 to time have been constructed with the view of explaining 

 their mode of formation. Just as the nature of the coral 



