Feb. 28, 1889] 



NATURE 



425 



by sea- water, and then there results the formation of coral reefs 

 and vast calcareous deposits at the bottom of the ocean. There 

 may be an almost exact balance between these processes. And 

 again, there may be more solution than secretion, as, for instance, 

 in the red clay areas, which occupy the deepest parts of the 

 ocean, and in some coral-reef lagoons. 



What is the nature of the foundations of these coral islands, 

 surrounded as they sometimes are by an ocean miles in depth ? 

 Why have some elongated reefs no lagoons ? Why have most 

 of the lagoons of the smaller atolls been filled up? Why is the 

 circle of land or reef in the perfect atolls only, at most, a few 

 hundred yards in diameter? What is the origin of the lagoon ? 

 What relation exists between the depth of the lagoon, its area, 

 and the depth of the water beyond the outer reef ? How has the 

 dry land of these islands been formed, provided with a soil, a 

 fauna and a flora ? These appear to be the chief questions that 

 demand an answer from any theory of coral island formation. 



These coral formations are essentially structures belonging to 

 the great oceans and ocean basins. They are dots of land within 

 the oceanic areas that might be compared or contrasted with the 

 small salt lakes which are scattered over the surface of the con- 

 tinental lands. A rapid survey of some of the more general 

 phenomena of the great oceans may, then, lead to a better 

 appreciation of the problems connected with coral reefs. 



The great ocean basins occupy over two-thirds of the earth's 

 surface, and have a mean depth of over two miles. The central 

 portions of these basins, called the abysmal regions, occupy about 

 one-half of the earth's surface, and have a mean depression below 

 the general level of the continents of over three miles. The 

 abysmal regions are vast undulating plains, sometimes rising to 

 less than two miles from the surface o I l he sea, and again sinking 

 to four and five miles beneath it. Volcanic cones rise singly or 

 in clusters from these great submerged plains. When they shoot 

 above the level of the sea they form single islands, like Ascension 

 and St. Paul's Rocks, or groups, like the Azores, the Sandwich, 

 the Fiji, and the Society Islands. As might have been expected, 

 there are many more of these cones hidden beneath the waves 

 than rise above them. When the Challenger sounded along the 

 west coast of Africa, there was no suspicion that between her 

 stations she was sailing over submerged cones. Since then, 

 however, the soundings of telegraph ships have correctly mapped 

 out no less than seven of these peaks between the latitude of 

 Lisbon and the Island of Teneriffe. The depths on the summits 

 of these vary from 12 to 500 fathoms. On one of them, at 

 400 fathoms, two species of coral {Lophohelia prolifera and 

 Amphihelia oculata) were growing luxuriantly. Throughout the 

 ocean basins about 300 such submarine cones, rising from great 

 depths up to within depths of from 500 to 10 fathoms from the 

 surface, are already known, or indicated by soundings. 



All the physical agencies at work above the lower limit of wave 

 action tend to wear away and level down these cones, and thus to 

 form banks. Graham's Island, thrown up in the Mediterranean 

 in 1831, was 200 feet in height and three miles in circumference, 

 and was washed away in a year or two. The bank left on the 

 spot, at first very shallow, has now 24 feet of water over it. 

 Instances similar to this historical example must often have 

 happened in the great ocean basins. Again, the same agencies 

 produce wide banks around volcanic islands by washing away 

 and spreading out the materials of the softer rocks. Such banks, 

 with depths of less than 60 fathoms, are found extending many 

 miles seawards around some volcanic islands. 



On the other hand, all the deeply submerged summits are 

 continually being built up to the lower limit of wave action by 

 the accumulation of the remains of animals which live on them 

 and by the fall of shells upon them from the surface waters. In 

 the Solomon Islands, Dr. Brougham Guppy has shown that 

 there are upraised coral islands with central volcanic cones 

 covered with thick layers of marine deposits ; Christmas Island, 

 in the Indian Ocean, is another instance, and similar deposits 

 must now be forming over hundreds of submerged mountains. 

 In this way are foundations prepared for the true reef-buiiding 

 species, which only flourish in the shallower depths. 



The bulk of the water of the ocean has a very low tempera- 

 ture ; it is ice-cold at the bottom, even under the equator, but 

 on the surface within the tropics there is a relatively thin film 

 of warm water, with a temperature of from 70° to 84° Y . This 

 film of warm water is much deeper towards the western parts of 

 the Atlantic and Pacific than it is in the eastern, the reason for 

 this being that the trade winds, which blow continually from 

 the east, carry all the warm surface water to the westward, and 



draw up cold water from beneath along the western shores of 

 Africa and America to supply the place of that driven westward 

 at the surface. Consequently, there is, at times, a very low 

 temperature, and a great annual range of temperature, along 

 these western shores. This is more clearly shown by the 

 temperatures at 50 and lOO fathoms than by those at the surface. 

 There are no coral reefs along the western shores of Africa and 

 South America, a circumstance evidently connected with the low 

 temperature, wide range, and, more directly, with the food 

 supply, consequent on these conditions. It appears to be a con- 

 firmation of this view that, on the eastern shores of Africa, about 

 Cape Guardafui, from off which the south-west monsoon blows 

 for several months in the year, cold water is also drawn to the 

 surface, and there, likewise, are no coral reefs, though they 

 flourish to the north and south of this region. 



Coral reefs flourish in mid-ocean and along the eastern shores 

 of the continents, or wherever the coasts are bathed by the 

 warmest and purest currents of water coming directly from the 

 open sea. If we except Bermuda and one or two other outlying 

 reefs, where the temperature may occasionally fall to 66° or 64° F. , 

 it may be said that reefs are never found where the surface tem- 

 perature of the water, at any time of the year, sinks below 70° F. , 

 and where the annual range is greater than 1 2° F. In typical 

 coral reef regions, however, the temperature is higher and the 

 range much less. 



The food supply of the coral reef is derived from pelagic 

 oceanic organisms, which exist in the greatest variety and 

 abundance in the surface and sub-surface waters of the ocean. 

 These consist of myriads of Algse, Rhizopods, Infusorians, 

 Medusae, Annelids, Molluscs, Crustaceans, Ascidians, and fishes. 

 A very large number of these creatures, within the tropics, 

 secrete carbonate of lime from the ocean to form their shells and 

 skeletons, which, falling to the bottom after death, form the 

 vast oceanic deposits known as Pteropod and Globigerina oozes. 

 In falling to the bottom, they carry down some of the organic 

 matter that composed their living bodies, and thus are the 

 animals which live on the floor of the ocean chiefly supplied 

 with food. Here it may be remarked, incidentally, that the 

 abundance of life at depths of even over two miles is very 

 great. Our small dredges sometimes bring up over sixty 

 species and hundreds of specimens in one haul — of inverte- 

 brates and fishes, exclusive of the Protozoa. The pelagic 

 organisms above mentioned oscillate from the surface down to 

 about 80 or \oo fathoms, probably that stratum of the ocean 

 affected by sunlight, and they apparently descend further in regions 

 where the stratum of warm water has a greater depth. Many 

 of the forms rise to the surface in the evening and during calms, 

 and sink again in sunlight and during stormy weather. It is in 

 the evening and when it is calm that this swarming life is most 

 vividly forced on the attention by gorgeous phosphorescent dis- 

 plays. The lime-secreting organisms, like Coccospheres and 

 Rhabdospheres, Foraminifera, Pteropods, and other Molluscs, 

 are much more abundant, both in species and individuals, in the 

 warmest and saltest waters than elsewhere. I have estimated, 

 from tow-net experiments, that at least 16 tons of carbonate of 

 lime, in the form of these shells, exist in a mass of the ocean, in 

 coral-reef regions, one mile square by 100 fathoms in depth. If 

 we take this estimate, which I consider much below the reality, 

 and suppose one-sixteenth of these organisms to die and fall to 

 the bottom each day, then they would take between 400 and 

 500 years to form a deposit one inch in thickness. I give this 

 calculation more to indicate a method than to give even the 

 roughest approximation to a rate of accumulation of deposits. 

 The experiments were too few to warrant any definite deductions. 

 The great oceanic currents, moving westward at the rate of 

 several miles an hour, bear these shoals of pelagic organisms on to 

 the face of the reef, where millions of greedy mouths are ready and 

 eager to receive them. The corals and other organisms situated 

 on the outer and windward side of the reef receive the first and 

 best supply ; they are thus endowed with a greater amount of 

 energy, and grow faster and more luxuriantly there than on other 

 portions of the reef The depth at which there is the most con- 

 stant supply of this food is several fathoms beneath the surface, 

 and there, too, the corals are found in most vigorous growth. It 

 is only a relatively small quantity of this pelagic food that enters 

 the lagoon, the corals that there straggle on in patches being 

 largely supplied with the means of existence from the larvae of 

 reef- building animals. 



So many observations were made during the Challenger 

 Expedition on the pelagic fauna inside and outside reefs that 



