POLYPI. 141 



A barrier reef {fig* 69. r', r') is essentially similar to the atoll or 

 coral-island. It runs parallel with the shores of some larger island or 

 continent ; separated, however, from the laud, by a broad and deep 

 lagoon channel (??, n), and having the outer side as deep and 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 terras 

 " fringing reefs" {fig. 68. r, r), differ from the barrier reefs in having 

 a comj)aratively small depth of water on the outer side, and a nar- 

 rower 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 

 physiology. 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 calcareous skeleton of a Madrepore or Millepore may be found, it 

 is certain that it must have been developed within thirty fathoms of 

 the surface of the ocean. If it coats the summit of the lofty moun- 

 tains of Tahiti *, it must have been lifted above the sea by the eleva- 

 tion 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 subsidence of the foundation on which the living madrepore 

 once flourished. It is by these movements of upheaval and sub- 

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

 forms which characterise the extraordinary productions of the coral 

 animals. Atolls, according to this author, rest on land which has 

 subsided, ai'd 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 



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

 7000 feet above the level of the sea. 



