第28章

It appears, as will be shown in the ensuing chapter, that the action of the surf is favourable to the vigorous growth of the stronger corals, and that sand or sediment, if agitated by the waves, is injurious to them. Hence it is probable that a reef on a shelving shore, like that of Mauritius, would at first grow up, not attached to the actual beach, but at some little distance from it; and the corals on the outer margin would be the most vigorous. A shallow channel would thus be formed within the reef, and as the breakers are prevented acting on the shores of the island, and as they do not ordinarily tear up many fragments from the outside, and as every streamlet has its bed prolonged in a straight line through the reef, this channel could be filled up only very slowly with sediment. But a beach of sand and of fragments of the smaller kinds of coral seems, in the case of Mauritius, to be slowly encroaching on the shallow channel. On many shelving and sandy coasts, the breakers tend to form a bar of sand a little way from the beach, with a slight increase of depth within it; for instance, Captain Grey (Captain Grey's "Journal of Two Expeditions," volume i. page 369.) states that the west coast of Australia, in latitude 24 deg., is fronted by a sand bar about two hundred yards in width, on which there is only two feet of water; but within it the depth increases to two fathoms. Similar bars, more or less perfect, occur on other coasts. In these cases I suspect that the shallow channel (which no doubt during storms is occasionally obliterated) is scooped out by the flowing away of the water thrown beyond the line, on which the waves break with the greatest force. At Pernambuco a bar of hard sandstone (I have described this singular structure in the "London and Edinburgh Phil. Mag." October 1841.), which has the same external form and height as a coral-reef, extends nearly parallel to the coast; within this bar currents, apparently caused by the water thrown over it during the greater part of each tide, run strongly, and are wearing away its inner wall. From these facts it can hardly be doubted, that within most fringing-reefs, especially within those lying some distance from the land, a return stream must carry away the water thrown over the outer edge; and the current thus produced, would tend to prevent the channel being filled up with sediment, and might even deepen it under certain circumstances. To this latter belief I am led, by finding that channels are almost universally present within the fringing-reefs of those islands which have undergone recent elevatory movements; and this could hardly have been the case, if the conversion of the very shallow channel into land had not been counteracted to a certain extent.

A fringing-reef, if elevated in a perfect condition above the level of the sea, ought to present the singular appearance of a broad dry moat within a low mound. The author ("Voyage a l'Isle de France, par un Officier du Roi," part i., pages 192, 200.) of an interesting pedestrian tour round the Mauritius, seems to have met with a structure of this kind: he says "J'observai que la, ou la mer etale, independamment des rescifs du large, il y a terre UNE ESPECE D'EFFONCEMENT ou chemin couvert naturel. On y pourrait mettre du canon," etc. In another place he adds, "Avant de passer le Cap, on remarque un gros banc de corail eleve de plus de quinze pieds: c'est une espece de rescif, que la mer abandonne, il regne au pied une longue flaque d'eau, dont on pourrait faire un bassin pour de petits vaisseaux." But the margin of the reef, although the highest and most perfect part, from being most exposed to the surf, would generally during a slow rise of the land be either partially or entirely worn down to that level, at which corals could renew their growth on its upper edge. On some parts of the coast-land of Mauritius there are little hillocks of coral-rock, which are either the last remnants of a continuous reef, or of low islets formed on it. I observed that two such hillocks between Tamarin Bay and the Great Black River; they were nearly twenty feet high, about two hundred yards from the present beach, and about thirty feet above its level. They rose abruptly from a smooth surface, strewed with worn fragments of coral. They consisted in their lower part of hard calcareous sandstone, and in their upper of great blocks of several species of Astraea and Madrepora, loosely aggregated; they were divided into irregular beds, dipping seaward, in one hillock at an angle of 8 deg., and in the other at 18 deg. I suspect that the superficial parts of the reefs, which have been upraised together with the islands they fringe, have generally been much more modified by the wearing action of the sea, than those of Mauritius.

Many islands are fringed by reefs quite similar to those of Mauritius (I may give Cuba, as another instance; Mr. Taylor ("Loudon's Mag. of Nat.