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. [Illustration: 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.]