The animals of this group show great variety of form, size, and habit of growth, and by no means all of them are important as rock-makers. The solitary corals, which are widely distributed, even in the deep sea, are never sufficiently abundant to form deposits by themselves. The corals which do accumulate in great masses and are called "reef-builders," form compound colonies or stocks, consisting of hundreds and thousands of individuals. The adult corals are sedentary, but the newly hatched young are worm-like, free-swimming larvae. When the young animal has established itself in a suitable place, preferably upon a rock or other fixed foundation, it develops into a polyp, or fleshy sack, with rows of tentacles around the mouth, and then by budding or partial division (fission) gives rise to great numbers of other polyps, which are connected by a tissue common to them all. In this compound mass is secreted a skeleton of carbonate of lime, which reproduces the form of the colony and, in most cases, displays cells for the individual polyps. The great variety of form shown by these compound colonies is determined by the mode of budding or fission and the relative position of the newer to the older polyps.
Thus, some are like trees, others like bushes; some form flat, irregular plates, while others grow into great dome-like masses.
The reef corals have, at present, a restricted distribution, and can flourish only where several favourable conditions are found united. They are preeminently shallow-water animals and can live only in depths of less than twenty fathoms. They also require a high temperature, and they cease wherever the average temperature of the water for the coldest month is below 68° F.; this is the minimum, and for full luxuriance a higher temperature is necessary. Another requisite is sea-water of full salinity and uncontami-nated with mud; hence, few corals can live at the mouth of a river, which, even if it brings down no sediment, freshens the water and is thus fatal to the polyps. Another condition favourable to the growth of corals is the presence of ocean currents, not too rapid, which bring abundant supplies of food, and they flourish best in the broken waters of heavy surf, which gives the necessary oxygen and prevents the smothering of the polyps in the calcareous silt and debris of the reef.
In short, the reef corals are tropical, marine, shallow-water animals, and their reefs are widely spread throughout the warmer seas of the globe, but they do not always occur where we should naturally expect to find them.
A coral reef is not built, as many people imagine, by the industry of the polyps - these furnish the material by extracting lime salts from the water and forming solid skeletons; the actual construction is largely the work of the waves and other agencies.
Fig. 139. - Corals on the Great Barrier Reef of Australia. (Savile Kent).
The coral colonies are scattered over the sea-bottom, much like vegetation on the land, scantily in some places, thickly in others, and in still others they are absent. The waves, especially in storms, break up the masses of coral, which are much weakened by the borings of many kinds of marine animals, and the surf grinds them down to fragments of all sizes, from large blocks to the finest and most impalpable mud. The process is the same as with the ordinary rocks of the coast, only the material differs, and thus are formed boulders, pebbles, sand and mud, all of coral fragments. The many animals which feed upon coral greatly facilitate this work, partly by boring into the masses, partly by grinding the smaller fragments into fine powder. Considerable masses of calcareous debris are added by the shells and tests of the various animals which live on and about the reef, and the coral-like seaweeds, called Nullipores, contribute an important quota, while shell sand often makes up as much as one-half of the volume of a reef. All of this material is ceaselessly ground up by the waves, distributed by tides and currents, and brought to rest in quiet waters.
A single deposit of two or three inches in thickness has been observed to form between tides after a gale along the Florida reefs, and in storms the water is often discoloured and turbid for miles around the reef. The sea-water dissolves and redeposits CaC03, cementing the fragments into a firm rock, which, especially after exposure to the air, may become very hard.
Fig. 140. - Various forms of modern coral limestone. (Savile Kent).
By these processes several varieties of rock are formed, corresponding, in all but the material, to the ordinary marine deposits. In one form the standing and unbroken colonies are filled up with calcareous debris and enclosed in solid masses. This is perhaps the most important kind of rock, at all events, in many reefs, for the branching corals retain the shell sand and other calcareous debris and prevent the waves from washing it away. Reefs of this kind have many and deep holes penetrating them, where the colonies are not in contact and the sand has not filled up the interspaces. Coral conglomerate or breccia is a cemented mass of coral pebbles or angular pieces, or is made up of fragments of an older coral rock. Reef rock is the dense and solid mass formed by the cementing of the finer debris which accumulates in quiet water. It is important to notice that even under the microscope reef rock frequently shows no trace of organic structure, and is a definite proof that the absence of such structure is not a sufficient reason for denying the organic origin of a rock. The interior of growing masses which are still alive on the outside, and have never been broken up, may be so crystallized by the action of the sea-water that the organic structure is obscured or destroyed.
On the beach is formed a curious rock called oolite, which is made up of minute spherules of CaC03 cemented into a mass not unlike fish-roe in appearance. This is due to the deposition of CaCO3 from solution around tiny grains of calcareous sand, until the spherules are built up and cemented together.
The growth of coral ceases when the reef extends up a little above low-water mark, but the waves continue their work and throw up debris and build up a platform, upon which they establish a beach of calcareous sand. The latter may be further piled up by the winds into dunes and solidified by the cementing action of percolating rain-water. According to circumstances, the new platform may be an extension of the shore or an island like the Florida Keys.
Coral reefs are classed according to their relation to the shore, and are of three kinds. (I) Fringing reefs are those attached directly to the land, though the exposed part may be at some distance out from the shore and separated from it by a shallow channel with coral bottom. The width of a fringing reef is determined by the slope of the sea-bottom, being narrower on a steep grade, broader on a gentle one. (2) Barrier reefs are farther out from shore, to which the reef is parallel in a general way, and separated by a broad and often quite deep channel. The distinction between the two kinds of reefs is not very sharply drawn, for the same reef may be fringing in parts of its course and a barrier in others. Even at the present time barrier reefs are sometimes constructed on an enormous scale. A great barrier reef runs parallel to nearly the whole north shore of Cuba, while the barrier reef of Australia, the largest known, extends, with some breaks, for over 1200 miles along the northeast coast of Australia, from which it is distant 20 to 80 miles; its breadth varies from 10 to 90 miles, though but little of this width is exposed above water; its sea-face is in some places more than 1800 feet high (i.e. above the sea-bottom, not the surface). (3) Atolls are coral islands of irregularly circular shape, which usually enclose a central lagoon and frequently, as in the Pacific, rise from the profoundest depths.
The way in which such islands have been built up is still a subject of much controversy. No doubt, atolls have been formed in various ways, especially those which arise from small depths, but probably the most important method is by a slow subsidence of the sea-bottom, with which the growth of the reef can keep pace. Such subsidence is the only explanation of great thicknesses of coral rock, as of any other kind of shoal-water deposits. Borings in the Hawaiian Islands, and especially in Funafuti, an island of the South Pacific, have demonstrated the existence of immensely thick coral limestones formed in the modern period.
Coral reefs in shoal water frequently have gentle slopes, but those which rise from the deep sea have very steep faces, sometimes as much as 65 °, and thus reefs may occur as lens-shaped areas, or steep-sided masses of limestone, in which stratification is very obscure, or absent, in the midst of well-stratified fragmental sediments.
A process has been observed in the closed lagoons of certain atolls which is significant as throwing light upon a very difficult problem, that of the formation of dolomite or magnesian limestone. In the closed lagoon, shut off entirely from the sea, the isolated body of sea-water becomes considerably concentrated by evaporation. All sea-water contains chloride of magnesium (MgCl2), and this percolating into the coral rock, by double decomposition with CaC03, forms MgC03. The change occurs more readily when the CaC03 is in the form of aragonite, as is the case in many shells and corals.
It is not known just how important a part is played by chemical precipitation in the formation of marine deposits, but probably a greater one than has been generally supposed. Rivers which bring in quantities of CaC03 in solution may so overload the sea with this substance (for sea-water will dissolve little of it) that more or less is precipitated in the neighbourhood of the land. On the coast of Asia Minor, for example, are large areas of sandstone and conglomerate, formed within recent times by the precipitation of CaC03 in masses of sand and gravel, binding them into hard rock. Similar examples are known elsewhere. There is also some reason to believe that the decay of marine animals evolves sufficient carbonate of ammonia to convert the sulphate of lime into the carbonate by double decomposition, and to precipitate the latter in some quantity.