84 THE GREAT BARRIER REEF. 



the totally uncompromisino; attitude towards the subsidence theory that is assumed by Dr. 

 Murray. 



The detailed exposition of Dr. Murray's views as epitomised by Dr. Bonny is herewith 

 submitted : — 



" Very nearly all oceanic islands, other than coral atolls, are now known to be of volcanic origin. Hence it is 

 probable that the foundations of the latter are volcanic rocks, and not those of an ancient pre-existent land. As shown 

 by the soundings of the Tuscarora and ChaUetiger, numerous submarine elevations exist which rise from depths of 2,000 

 to 3,000 fathoms to within a few hundred fathoms of the surface. The upper waters of the ocean (to a depth probably 

 of about 100 fathoms) teem with organisms, calcareous and siliceous, such as algre, protozoa, hydrozoa, moUusca, and 

 other members of the animal kingdom : these are drifted by the currents from place to place ; by these the reef-building 

 corals are supplied with food. It has been estimated as the result of experiment, that a mass of ocean water one mile 

 square and 100 fathoms deep, contains more than sixteen tons of carbonate of lime. After death the skeletons of these 

 organisms are showered down upon the bed of the ocean. In water which exceeds some 800 or 900 fathoms 

 in depth these remains are more or less affected by the solvent power of the carbonic acid present in the water, but at 

 less depth they accumulate. Thus, any submarine bank which rose within the above-named depth would be brought 

 nearer to the surface, and its upper part, as the water above it shallowed, would be colonised by larger pelagic organisms, 

 these after death would augment by their remains the increasing pile of material, which at last would arrive within the 

 bathymetrical zone in which the recf-building corals can live^ and the formation of an atoll would commence. 



"As already pointed out by Mr. Darwin, the corals on the outer margin of a bank grow vigorously, while the diminu- 

 tion of food and the increase of sediment tend to check the development of those in the inner part. Thus, while the 

 reef is still several fathoms below the surface, the corals in the central part are placed at a disadvantage, which becomes 

 greater as they are left behind in the upward race by their neighbours. In a small reef, the periphery for the supply of 

 food to the interior is relatively large, thus the lagoons in small atolls are also small and soon filled up, while long and 

 narrow banks have no lagoons. As the reef becomes larger the conditions become more favourable to the formation of 

 lagoons, for (as is shown by experiment) the lagoon of such an atoll is less rich in pelagic life than the exterior water. Thus 

 growth is checked, many species of coral die, and their calcareous ' skeletons ' are exposed to the solvent action of sea- 

 water. When the water outside becomes too deep for reef-building corals to live, the debris from the existing reef, aided 

 l)y the accumulation of organisms, forms a talus at the foot of its submarine cliffs, and thus the reef spreads slowly out- 

 wards, ' like a fairy ring,' on foundations to which its own materials have contributed. The lagoon-channels have in many 

 cases been subsequently formed by the solvent action of sea-water, and the islets in the lagoon-channels are parts of the 

 original reef still left standing. When the reefs rise quite up to the surface and are nearly continuous, there is little coral- 

 growth in the lagoon or its channels ; where the outer reefs are much broken up, the growth is relatively abundant. 



" At the Admiralty Islands, on the lagoon side of the islets of the barrier reefs, the trees are found overhanging the 

 water, and in some cases the soil was washed away from their roots. It is a common observation in atolls that the islets 

 on the reefs are situated close to the lagoon shore. These facts point out the removal of matter which is going on in the 

 lagoons and lagoon-channels. 



" Elevation, not subsidence, is to be expected in a volcanic region, as there is an a priori reason for attributing the 

 ])henomena of coral reefs — as resting on volcanic foundations — to elevation rather than subsidence. The former 

 hypothesis appears to Mr. Murray to accord with all the facts indicated by the published charts of coral-reefs, and thus is 

 considered by him preferable to the latter." 



Mr. Murray's general conclusions may be briefly enumerated as follows : — 



