. Elements of geology : a text-book for colleges and for the general reader. Geology. THEORIES OF BARRIER AND CIRCULAR REEFS. 151 than the upward building of the coral-ground; that every reef com- mences as a fringing reef, but, in the progress of subsidence, was con- verted first into a barrier and finally into an atoll. For, as the vol- canic island went down, the corals would build upward on the same spot; and as the island would become smaller and smaller, and the corals would grow faster on the outer side of the reef, where they are exposed to the breakers, it is evident that the reef wou
. Elements of geology : a text-book for colleges and for the general reader. Geology. THEORIES OF BARRIER AND CIRCULAR REEFS. 151 than the upward building of the coral-ground; that every reef com- mences as a fringing reef, but, in the progress of subsidence, was con- verted first into a barrier and finally into an atoll. For, as the vol- canic island went down, the corals would build upward on the same spot; and as the island would become smaller and smaller, and the corals would grow faster on the outer side of the reef, where they are exposed to the breakers, it is evident that the reef would become sepa- rated from the island by a ship-channel, and thus become a barrier. Finally, when the island disappears entirely, the reef, still building upward, would become an atoll. These changes are represented in the accompanying section (Fig. 125). As the changes are relative, they may be represented either by the land sinking or the sea-level ris- ing ; for the sake of convenience we use the latter. In the figure, I" I" represents the sea-level when the reef was & fringe, I' I' when it was a barrier, and I I the present sea-level, when it has become an atoll. The ship-channel and the lagoon, though always lower, rise pari passu with the reef proper. This is the- result partly of the growth of placid-water species of corals, and partly of the drifting of coral debris from the reef, and detritus from the volcanic island. It is seen that the corals do not build a vertical wall, and therefore that the atoll is always smaller than the coast-line of the original island. Consequently, if the subsidence continues, a typical atoll is changed into a small closed lagoon, and, finally, into a lagoonless island. These, therefore, indicate the deepest Fig. 125. Evidences.—1., This theory accounts for all the more obvious phe- nomena of atolls, such as their irregular circular form, their size, the steepness of their outer slopes, etc. 2. Every stage of gr
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