The problem is complicated by the fact that ordinary reef corals die out at a depth of 50 fathoms or so. Now 50 fathoms is a mere nothing compared to the depths from which the Pacific atolls rise, and is only a quarter the depth often found within a few hundred yards of the Red Sea reefs. How then to account for the building of reefs in deep water?
One suggestion was that atoll rings were formed by the growth of a mere cap of coral round the edge of the craters of huge submarine volcanoes. But that postulates far too large a number of such immense volcanoes[56], and the early stages of these formations have not been found. Darwin’s hypothesis was hailed with joy as the obvious solution, and held the field against all rivals for many years. Briefly it is that corals formed a reef by direct growth in shallow water on the coast of an island, forming a fringe thereto in the way explained above. Now is postulated one of those great, slow earth movements such as have very often occurred in the past and are occurring at the present day. In this case the island is to sink slowly, at such a rate that the reef grows upwards as fast as it is submerged. The result is obviously a mass of corals of a thickness equal to the total sinking movement of our island, though every individual coral grew while in water under 50 fathoms deep.
When our island is half submerged the fringing reef has become a barrier, when wholly gone the reef ring remains enclosing an empty lagoon, and is the only mark of the grave of a drowned island. Thus Darwin’s theory has the further merit of referring the two forms of reef, barrier and atoll, to one common cause, the sinking of the land. But we have no idea of how the original islands were formed in such numbers, and many believe that no such vast sinking of the ocean basins has occurred since they were formed. Also, if solution be ignored, it is difficult to see why, as the island sank, coral growth did not close in over the submerged land, and so form a vast reef flat instead of leaving a lagoon up to 50 fathoms deep.
To settle the matter an expedition was sent to a typical atoll, Funafuti, and a boring 1200 feet deep was made to find out what the interior of the reef is made of. The material brought out of the bore hole has been carefully examined by experts, and reported to consist of the remains of exactly similar corals to those found near the surface, and this result was taken by one or two geologists as complete vindication of Darwin’s theory. But apart from the extreme difficulty of the identification of all coral species, especially those which have been subject to partial crystallisation and so on, one remembers that a considerable part of the foundations in deep water are formed of corals which have fallen down the steep slope from the growing reef above, so that their presence buried a thousand fathoms deep proves nothing, while the boring at Funafuti only went to about 200 fathoms.
Diagram 6. Original elevation A of sea bottom shaded, B₁-B₄ additions formed by growth, C₁-C₃ slopes of coral &c. fallen from above. The thick line of C₃ is outline of section of the atoll mass resulting.
After all it is easier to imagine that the atoll grew up from the bottom of the deep sea. The only postulate is a chance elevation on the sea bottom. On such elevations it is found that the remains of marine organisms, including deep sea corals (as distinct from reef builders), tend to accumulate much more rapidly than on the floor of the surrounding depths. The elevation is consequently slowly but surely raised, and the higher it grows the more rapid the accumulation, until at last reef corals obtain a footing forming a cap or pinnacle reaching to the surface. From this masses of coral, sand, stones or large boulders, are always falling on to the foundation slopes, forming successive sloping layers indicated by the dotted lines, upon which fresh growth takes its rise[57]. When the coral reef has become of some breadth (and atoll rings may be 30 miles or more across) a boring at the edge might descend for a thousand fathoms and never meet with the original foundations, but would pass only through recent corals fallen from the shallow zone.
We should expect to find a continuous surface of coral at sea level. As a matter of fact there is a broad lagoon, generally of considerable depth, one or two gaps through the encircling reef giving communication with the open ocean. This is the natural result of the causes described when dealing with the boat channel of a fringing reef; it is the same thing on a much larger scale. Seeing that the rate of growth of coral masses is always only the excess of growth over destruction and solution, the presence of growing corals is no evidence against the fact that the lagoon shores may be undergoing destruction, and that such coral growth as is present may add nothing to the inner sides of the reef in the end. No more does the accumulation of great quantities of mud prove that the lagoon will in time be quite filled in. Mud and sand[58] are but stages in the destruction of coral rock, and its presence where that process is going on is to be expected. An abnormal tide, a shift of the currents, and vast quantities are swept out through the gaps in the reefs. My home on the Red Sea is beside a large landlocked lagoon in which coral gardens of great luxuriance, whence collections of many species can be procured, are frequent. Spite of this, the evidence is as clear as possible that its shores and islands are undergoing rapid denudation, and its reefs are being cut down by currents to banks below water level. As in the Red Sea the level rarely alters by more than a foot once in the twenty-four hours, and often the rise or fall is much less, the action of tidal currents is at a minimum, yet even so they produce well-marked effects.
Barrier reefs may be formed from fringing reefs by the enlargement of the boat channel, while the reef is extending seawards.
The island of Zanzibar, 60 miles long by 20 wide, and 20 miles from the mainland of Africa, seems to be a part of the East African barrier system, and it certainly was separated from the mainland by the destruction of the intervening land; the shallow dividing channel being full of shoals and sandbanks formed by cutting down of islands. The fauna of Zanzibar, including leopards, serval cats, &c., can be accounted for in no other way. The Great Barrier of Australia, a thousand miles long, is the same thing on a vastly greater scale. But, as described in the next chapter, the Barrier system of the Red Sea is quite another thing, and its mode of formation may possibly be unique in the world.