POLYPI. 91 



steep as in the Lagoon Islands. Here likewise the skeletons of the 

 Zoophytes, of which the reef is composed, are found on the outer 

 precipitous wall as deep as sounding line can reach. 



The third class of coral productions which Mr. Darwin terms 

 '' Fringing Reefs " {fg. 48. r, r), differ from the Barrier Reefs in 

 having a comparatively small depth of water on the outer side, and 

 a narrower and shallower lagoon channel between them and the 

 main land. 



These differences in the characters of the wonderful fabrications of 

 the coral animalcules are explicable by the following facts in their phy- 

 siology. The animals of the Porites and Milleporce cannot exist at a 

 greater depth than twenty or thirty fathoms ; beyond this the stimuli 

 of light and heat derived from the solar beams become too feeble 

 to excite and maintain their vital powers. On the other hand, their 

 tissues are so delicate, that a brief direct exposure to the sun's rays 

 kills them ; and unless they are constantly immersed in water or beaten 

 by the surf, they cannot live. Thus, in whatever position the calca- 

 reous skeleton of a Madrepore or Millepore, may be found it is certain 

 that it must have been developed Avithin thirty fathoms of the sur- 

 face of the ocean. If it coats the summit of the lofty mountains of 

 Tahiti*, it must have been lifted above the sea by the elevation of the 

 rock on which it was originally deposited. If it is brought up from 

 the depth of 200 or 300 fathoms, as at Cardoo Atoll or Keeling Atoll, 

 it must have been dragged down to that depth by a gradual sub- 

 sidence of the foundation on which the living madrepore once 

 flourished. It is by these movements of upheaval and subsidence 

 of the earth's crust, that Mr. Darwin explains the different forms 

 which characterise the extraordinary productions of the coral animal. 

 The Atolls or Lagoon Islands, according to this author, rest on land 

 which has subsided, and part of which was once dry. Barrier' reefs 

 indicate the islands or continents, which they encircle, to be the 

 remains of land now partly submerged, and perhaps in progress 

 towards final disappearance. Fringing reefs^ on the contrary, indicate 

 either that the shores are stationary, or that they are now rising, as 

 in most of the Sandwich Islands, where former reefs have been raised 

 many yards above the sea. 



Elizabeth Island, which is eighty feet in height, is entirely com- 

 posed of coral-rock. The coral animals, thus progressively lifted 

 above their element, are compelled to carry on their operations 

 more and more remote from the former theatres of their constructive 

 energies, but cannot extend deeper than their allotted thirty fa- 

 thoms : the direction of their submarine masonry is centrifugal 



* Mr. Stutchbury here found a regular stratum of semifossil coral at 5000 ^and 

 7000 feet above the level of the sea. 



