April 9, 1903 J 



NA TURE 



547 



ON THE FORMATION OF BARRIER REEFS 



l\D OF THE DIFFERENT TYPES OF 



ATOLLS. l 



THE results here presented are based upon observations 

 carried on during the past twenty-five years in Florida, 

 the Bermudas, Bahamas, Cuba, Jamaica, and the West 

 Indies in the Atlantic. They include in the Pacific the 

 Galapagos, the Hawaiian Islands, the Great Barrier Reef of 

 Australia, the Fiji Islands, and the Coral Reefs and Islands 

 of the tropical Pacific, from the Marquesas to the Paumotus, 

 the Society Islands, the Cook Archipelago, Niue, the Tonga, 

 Ellice, Gilbert, and Marshall Islands, the Carolines and 

 Southern Ladrones, and the Maldives, in the Indian Ocean. 



Recognising that Darwin's theory did not explain the 

 conditions observed, ray reports were limited to descrip- 

 tions of the different types of Coral Reefs and of the causes 

 to which they probably owed their formation, and no 

 attempt was made to establish any independent general 

 theory. 



Beginning with the Barrier Reefs, we find that those of 

 Fiji, the Hawaiian Islands, and the West Indies usually 

 flank volcanic islands and are underlaid by volcanic rocks. 

 Those of New Caledonia, Australia, Florida, Honduras, and 

 the Bahamas are underlaid by outliers of the adjoining 

 land masses, which crop out as islands and islets in the 

 very outer edge of the Barrier Reefs. Some of the Barrier 

 Reefs of the Society Islands, of Fiji, and of the Carolines, 

 show that the wide and deep lagoons, separating them from 

 the land mass, have been formed by erosion, from a broad 

 fringing reef flat. Encircling reefs, such as characterise 

 especially the Society Islands, hold to their central island 

 or islands the same relation which a Barrier Reef holds 

 to the adjoining land mass. Denudation and submarine 

 erosion fully account for the formation of platforms upon 

 which coral reefs and other limestone organisms may build, 

 either barrier or encircling reefs, or even atolls, rising upon 

 a volcanic base, of which the central mass may have dis- 

 appeared as in Fiji, the Society and Caroline Islands. 



We may next take the type of elevated islands of the 

 Paumotus, the Fiji, the Gilbert, and the Ladrones, many- 

 composed only of Tertiary limestones, others partly of lime- 

 stone, and partly volcanic. We can follow the changes 

 from an elevated island, like Niue, or Makatea in the Pau- 

 motus, to an island like Niau, through a stage like Rangiroa 

 to that of the great majority of the atolls in the Paumotus. 

 The reef-flats and outer reefs flanking elevated islands hold 

 peculiar relation to them ; they are partly those of Barrier 

 Reef and partly of Fringing Reef. We may also trace the 

 passage of elevated plateaux like Tonga, Guam, and islands 

 in Fiji, partly volcanic and partly limestone, to atolls where 

 only a small islet or a larger island of either limestone or 

 volcanic rock is left to indicate its origin. Atolls may also 

 be formed upon the denuded rim of a volcanic crater as at 

 Totoya and Thombia in Fiji, as well as in some of the 

 volcanoes east of- Tonga. 



In the Ellice and Marshall group and the Line Islands, 

 are a number of atolls, the underlying base of which is not 

 known and where we can only follow the formation of the 

 land rim of the atoll, as far as it is due to the agency of 

 the trades or of the monsoons in constantly shifting the 

 superficial material (prepared by boring organisms) which 

 goes to form its rim. Many of the atolls in the Pacific are 

 merely shallow sinks, formed by high sandbanks, thrown up 

 around a central area. 



Throughout the Pacific, the Indian Ocean, and the West 

 Indies the most positive evidence exists of a moderate, recent 

 elevation of the coral reefs. This is shown by the horses, 

 pinnacles, and undermined masses of modern or Tertiary- 

 limestone left to attest it. The existence of honeycombed 

 pinnacles of limestone within the lagoons of atolls, as 

 shoals, islands, or islets, shows the extent of the solvent 

 action of the sea upon land areas, having formerly a great 

 extension than at the present day. Signs of this solvent 

 action are to be seen everywhere among coral reefs. Atmo- 

 spheric denudation has played an important part in re- 

 ducing elevated limestone islands to the level of the sea by 

 riddling them with caverns and by forming extensive sinks, 

 often taken to be elevated lagoons. 



1 By Alexander Agassi?, For. Men. R.S. Read at the Royal Society 

 March 19. ' " 



NO. 1745, v O L - 67] 



Closed atolls can hardly be said to exist ; Niau in the 

 Paumotus is the nearest approach to one, yet its shallow 

 lagoon is fed by the sea through its porous ring. Sea water 

 may pass freely into a lagoon at low tide over extensive 

 shallow reef flats where there are no boat passages. 1 In- 

 land area of an atoll is relatively small compared to that 

 of the half-submerged reef flats. This is specially the . ase 

 in the Marshall Islands and the Maldives where land areas 

 are reduced to a minimum. 



The Maldivian plateau with its thousands of small atolls, 

 rings, or lagoon reefs, rising from a depth varying from 

 twenty to thirty fathoms is overwhelming testimony that 

 atolls may rise from a plateau of suitable depth, wherever 

 and however it may have been formed and whatever may 

 be its geological structure. On the Yucatan plateau similar 

 conditions exist regarding the formation of atolls, only on 

 a most limited scale. 



The great coral reef regions are within the limits of the 

 trades and monsoons and areas of elevation, with the ex- 

 ception of the Ellice and Marshall Islands and some of the 

 Line Islands. The extent of the elevation is shown by the 

 terraces of the elevated islands of the Paumotus, Fiji, Tonga, 

 Ladrones, Gilbert, and West Indies, or by the lines of cliff 

 caverns indicating levels of marine erosion. 



In the regions I have examined the modern reef rock is 

 of very moderate thickness, within the limits of depth at 

 which reef builders begin to grow and within which the 

 land rims of atolls or of Barrier Reefs are affected by 

 mechanical causes. This does not affect the existence of 

 solitary deep sea corals, of extensive growths of Oculina or 

 Lophohelia at great depths, or in any way challenge the 

 formation of thick beds of coraliferous limestone during 

 periods of subsidence. 



The Marquesas, Galapagos, and a few islands in the 

 Society and West Indies have no corals, although they are 

 within the limits of coral areas. Their absence is due to 

 the steepness of their shores and to the absence or crumbling- 

 nature of their submarine platforms. Coral reefs also can- 

 not grow off the steep cliff faces of elevated, coraliferous 

 limestone islands. 



Corals take their fullest development on the sea faces of 

 reefs ; they grow sparingly in lagoons where coralline alga- 

 grow most luxuriantly. Nullipores and corallines form an 

 important part of the reef-building material. 



UNDERGROUND WATERS. 



" T'HE Motions of Underground Waters " is the title of 

 *■ an essay by Mr. Charles S. Slichter, and it is issued 

 as No. 67 of the Water Supply and Irrigation Papers of 

 the United States Geological Survey. The author, in the 

 first place, discusses the origin and extent of underground 

 waters, remarking that these are included only in the zone 

 of saturated rocks, the surface of which is known as the 

 ■water table or water plane. The lowest depth at which 

 ground waters can exist is regarded as approximately six 

 miles. The region above this limit is distinguished as the 

 zone of fracture, for in it pressures and stresses result in 

 the breaking of the rocks. Below, all cavities and pores 

 in the rock are completely closed. The amount of ground 

 water within the crust of the earth is estimated to be nearly 

 one-third the amount of the oceanic water, and to be 

 sufficient to cover the entire surface of the earth to a uniform 

 depth of from 3000 to 3500 feet. But these " waters under 

 the earth " are, of course, only recoverable in useful quanti- 

 ties at limited depths ; even the thermal springs arise from 

 a level much above the geologic limit of depth. 



Attention is directed to the fact that water is found in 

 notable quantities in crevices of schists and gneisses, as in 

 the St. Gothard tunnel ; but the greater part met with in 

 rocks is stored up in the minute pores and openings between 

 the rock particles themselves, in sands, sandstones and 

 limestones, in clay loams, while even the strongest rocks, 

 such as the Montello granite, are measurably porous. 



The author then discusses the cause and rate of move- 

 ment of water through the strata, according to the size of 

 the pores, the pressure and the temperature, the flow being 

 noticeably greater for high than for low temperatures. 

 This subject is illuslrated by microscopic sections of rocks, 



