lieved to be the true explanation of the origin of such reefs. It is, indeed, very probable that some reefs and atolls have been formed in this way, the mistake made by Darwin, and still more unreservedly by Dana, was in assuming that all barrier-reefs and atolls had been so formed, and that every such reef might be considered as a proof of subsidence. More recently Professor Semper, from a study of the Pelew Islands (1863); L. Agassiz and Leconte, of the Florida Reefs (1851-66); Dr. J. Murray, from the "Challenger" Expedition (1880); A. Agassiz, writing on the Florida Reefs (1882); Dr. Guppy, on the Solomon Islands (1884-87); and Mr. J. C. Bourne, on the Coral Formations Fig. 69. Diagram to illustrate Dr. Murray's theory of the basis of Coral-reefs. a a. The original bank. bb. Deposit of organic débris. ss. Level of the sea. cc. Coral. of the Indian Ocean (1888), have all dissented from Darwin's conclusions. These writers have pointed out the following facts among others : 1. The three kinds of reefs,-Fringes, Barriers, and Atolls, frequently occur in the same group of islands. 2. That all these kinds also occur within regions which are known to have been recently elevated, as, for instance, the Florida region and that of the Solomon Islands. 3. That recent observations have shown how submarine banks and ridges may be built up by accretion till they form foundations for coral-reefs. The careful examination of the reefs of Florida and the adjacent islands, by Professor A. Agassiz, leaves little doubt that they are based upon the great shell-banks which exist in those seas, and which are still being formed (see p. 279). Fig. 69 shows how a submarine bank, ridge, or eminence may be heightened by the gradual accumulation of organic débris till its surface is brought within the depth of water in which reef-corals can live, and how an atoll-like reef may then be built upon it. Ocean shoals are much more numerous than was suspected in Darwin's time, and every submarine eminence is being heightened by the accumulation of organic débris on its surface. Mr. Murray has estimated that in tropical Fig. 70. Chart of Macclesfield Bank (after Capt. Wharton). seas a square mile by 100 fathoms deep contains over sixteen tons of carbonate of lime in the shape of living organisms; and besides this there are all the creatures which grow or creep on the surface of the shoal itself. Again, in volcanic regions cinder-cones often form temporary islands, which are gradually worn down by the waves till only a submarine shoal remains, and this would form a suitable foundation for a coral island. Finally, Captain Wharton has called attention to certain submarine banks which present the appearance of being atolls in the process of formation.' Thus the Macclesfield Bank in the China Sea is a large oval area 70 miles in length, the central part of which is covered by 40 or 50 fathoms of water, but all round its rim are a series of growing coral-reefs, some of which rise to within 10 fathoms of the surface (see fig. 70). The Tizard Bank in the same sea presents similar features, but in this case some of the reefs have actually reached the surface, while others are still 4 or 5 fathoms deep. 2 Dr. Coppinger, describing the Admirante Islands in the Indian Ocean, says they stand on an oval-shaped bank which has a raised rim and a central depressed area, and he remarks that several of the islands present positive evidence of elevation. The African Islands exhibit similar features. Here, therefore, we have banks with the aspect of submerged or imperfect atolls which bear direct evidence of upheaval, not subsidence. pre The Tonga Islands in the Pacific afford instances of raised coral-limestones resting on stratified volcanic ash and tuff. Falcon Island, which is one of the group, and was described on p. 45, shows how this basis may be pared; and Mr. J. J. Lister has shown that elevation has been in progress for a great length of time, raising coralreefs on Eua and Vavau to heights of 500 and 1,000 feet above the sea.3 Given a foundation, we only need an explanation of the special coral growth on its border to understand the formation of an atoll. This is not difficult. All writers (Darwin included) agree that the corals which establish themselves on the outer parts of a shoal are likely to grow more vigorously than those on the inner parts, because they get a more abundant supply of food in the sweep of the ocean currents. The currents themselves too exert a directing influence: several observers, notably Semper, Bourne, and Hickson, lay stress on this influence, and state that corals will not flourish in a very strong current (6 or 7 miles an hour), nor in still water, but grow "Nature," Feb. 23, 1888, p. 393. 2 "Cruise of the Alert,'" p. 225. 3"Quart. Journ. Geol. Soc.," vol. xlvii. p. 590. luxuriantly in places where a moderate current flows over them. Such conditions exist on the outer slopes of a submarine bank more than over its central part. Moreover, when once the outer reefs have grown up so far as to come within the influence of wave action, other causes come into play; broken fragments and dead coralblocks are piled on these reefs, while fine sand and sediment are carried inward over the lagoon area, and are kept in movement by every tide. This exerts a great repressive influence on the coral growths of the central space; and this repression is aided by the boring and excavating action of certain worms, Molluscs and Echini, which attack dead coral, and possibly also (as Murray suggests) by the actual solution of some of the dead coral. Sediment in motion, however, seems to be the chief check to coral growth, and where the scour of the tide is not very great the lagoon is sometimes being choked up with sediment, as in the CocosKeeling Atoll. Putting all the preceding considerations together, we arrive at the conclusion that Darwin's theory cannot be accepted as a general explanation of Atolls and Barrierreefs, and that the inferences drawn from it as to the great thickness of coral-rock beneath some reefs cannot be entertained without further evidence. Both Professor Agassiz and Dr. Guppy truly remark that Darwin and Dana were not justified in assuming the submarine slope outside a coast-line to be a prolongation of the exposed land-slope. Such a mode of reasoning is very fallacious, and cannot lead to accurate results. Experience has shown that submarine slopes are very irregular, and are often broken into a series of gentle inclines with short, steep slopes between them. It has also been found that there are such things as submarine barrier-reefs as well as submarine atolls, that is to say, wall-like reefs which rise from ledges where the corals are forced to grow upward, being limited on the one side by deep water and on the other by unstable sand or mud derived from a reef nearer the shore which is exposed to wave-erosion.1 1 See Guppy, "Proc. Roy. Soc. Edin.," 1886, p. 880; JukesBrowne and Harrison, Quart. Journ. Geol. Soc.," vol. xlvii. p. 204; and Captain Wharton, "Nature," 1888, p. 395. CHAPTER XVI. DEEP-SEA AND OCEANIC DEPOSITS. WE have now to consider the deposits which are being formed in the great oceanic depressions which lie outside the shelf or zone of comparatively shallow water which border the continental areas. These deposits may be classified as follows: 1. Blue muds. 2. Glauconitic marls and sands. 3. Calcareous oozes. 4. Red clays. 5. Siliceous oozes. 1. Blue Muds.-It was mentioned on p. 267 that the finest part of the sediment derived from the waste of land-surfaces is carried for a certain distance into the seas and oceans. This sediment forms a layer of blue mud, and there is generally a band of such mud immediately outside the sands which border the coasts of the great continents. It often extends to considerable depths, but seldom covers a large area except where great rivers discharge into wide bays or gulfs. Thus in the Arabian Sea and in the Bay of Bengal blue mud extends to a distance of 700 miles from land and down to depths of more than 2,000 fathoms, and covers an area of about 1,700,000 square miles.1 Dr. Murray describes these muds as soft and reddish at the surface, but stiff, blue, and tenacious underneath. They contain much more organic matter than the more remote oceanic deposits, and it is the slow decomposition of this which, by reducing the higher oxides, gives the mass a 1 See map and description of the deposits in the Indian Ocean, by Dr. J. Murray, in the "Scottish Geographical Magazine, August, 1889. |