Voyages of Discovery and Survey. 327 



studied coral reefs that most coral polyps cannot support life where 

 the waters are charged witli muddy matter or fine detritus. We 

 have often had occasion to observe this fact around Jamaica, As 

 the Australian coast is approached, notwithstanding that it is pro- 

 tected from the ocean breakers by the outer reefs, there is still 

 enough abrasion by the sea to produce detritus, adding to that which 

 may "be brought down by the rivers. These are known to be of so 

 little importance that water is scarce along the whole coast bounded 

 by the coral barrier. Indeed if it were otherwise the barrier would 

 be broken through, or rather would never have been formed where 

 the muddy waters of large rivers interfered with the coral growth. 



Towards the coast, therefore, there would be conditions antagonist 

 to the growth of the reef-making corals, which would gradually cease 

 towards the clear water out.ide the reefs. Though interfering with 

 coral life, they would necessarily have no influence on the extension 

 of the coral pebbles, sands, and finer calcareous mud towards the 

 shore, where all such would be sorted and deposited in the usual 

 manner. Detritus from the land would mingle with the calcareous 

 sediments towards the coast, and especially when any was forced out 

 of river-mouths, where, and in other sheltered situations, those de- 

 tritus-collecting plants, the mangrove-trees, appear to be, from the 

 descriptions of Mr Beete Jukes, very common. 



Respecting the volume of this accumulation, including any detritus 

 obtained from the shores, chiefly, it would appear, granitic, and even 

 supposing it to repose on a sea-bottom gradually sloping to the great 

 depths fo^und immediately outside the great barrier reef, we have to 

 measure it by no small amount of cubic miles. If to this we add the 

 mass of similarly -formed matter now constituting the atolls or lagoon- 

 islands, the encircling reefs and the shore reefs of the Indian and 

 Pacific Oceans, and now constanly increasing, the various coral and 

 other germs settling and flourishing wherever they find the conditions 

 suited to them, we have an immense mass of carbonate of lime 

 transformed from a state of solution to that of a solid by the agency 

 of animal life, adding most materially to the rocks now accumulating 

 in the tropical regions of our globe. 



When we consider that heavy breakers are favourable to, and do 

 not impede, the growth of certain corals, indeed such situations must 

 prevent the attacks of many coral-feeding animals which would other- 

 wise crop down the polyps and their fragile cells ; and that the germs 

 of the surf-loving corals are floating about ready to settle wherever 

 the sea is clear enough, and the temperature and other general con- 

 ditions are suited to their growth, there seems little limit to their 

 extension where such conditions obtain. These existing, a reef is 

 formed, and the mechanical destruction of portions of it follow. If 

 the power of the polyps to secrete their harder parts be as a whole 

 greater than that of the surf to break them ofl',— the parts which can 

 bo broken off falling into deep water beyond the range of surf, and 



