rain water and atmospheric erosion leached out depressions in its central parts and finally the sea entered, forming a lagoon surrounded by a ring of detached islets of elevated limestone. In other cases the crater rims have washed away and a ring of coral reefs now marks the site of the old volcanic ridge.
According to Agassiz the coral reefs of to-day, in the Fiji Islands, form only a crust of moderate thickness upon a base of old limestone or volcanic rock. The present corals form only fringing reefs along the shores, and the contours of the atolls and barrier reefs are thus due to causes which acted at the time when the islands were elevated late in the Tertiary period.
In so great an archipelago as that of the Fijis with more than 270 islands there must be many details of reef formation the elucidation of which requires more prolonged study than Agassiz was enabled to devote to them in his visit of less than three months; for example, he was puzzled to explain the great thickness—1,000 feet and more—of the elevated limestones; for reef-corals do not grow at depths greater than about 120 feet. Could these enormous accumulations have been formed by coral reefs during a period of slow subsidence, as Darwin had assumed, or were they merely the talus of broken fragments which had rolled down the sea-ward sides from the outer edges of the reef, or were they formed by a slow accumulation of limestone fragments and shells of marine creatures other than corals which had lived upon the bottom more than a thousand feet beneath the surface and gradually built up a vast mass of limestone, as was the case with the great submerged Pourtalès Plateau off the Florida coast? He had in mind the fact that even in the richest coral reef regions the masses of broken shells and fragments of calcareous plants are commonly vastly greater than the bulk of the corals, for the corals grow only here and there over the limestone flats, and flourish luxuriantly only on their seaward slopes. Were such a reef to form during a long period of slow subsidence, and then become elevated above the sea we should find only an occasional coral here and there imbedded in a great mass of limestone. This is the appearance presented by some of these elevated limestone cliffs of the Pacific islands, while others appear to be walls of non-coral-bearing limestone capped above with a crust of corals. In many cases, however, the corals they once contained have disappeared and been replaced by calcite or dolomite. These elevated limestones soon become very hard when exposed to the atmosphere, for they become coated by a dense veneer which rings with a clinker-like sound when struck with a hammer. One sometimes finds shells of the giant clam, Tridacna, imbedded in this hard limestone and elevated above the sea, and yet the nacre of the shell is still white and polished, thus proving that the rock
