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. [2] Captain Grey’s “Journal of Two Expeditions,” volume i. page 369. [3] I have described this singular structure in the “London and Edinburgh Phil. Mag.” October 1841. 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[4] of an interesting pedestrian tour round the Mauritius, seems to have met with a structure of this kind: he says “J’observai que là, où la mer étale, indépendamment des rescifs du large, il y à terre _une espèce 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 elévé de plus de quinze pieds: c’est une espèce de rescif, que la mer abandonné, 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 Astræa and Madrepora, loosely aggregated; they were divided into irregular beds, dipping seaward, in one hillock at an angle of 8°, and in the other at 18° 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. [4] “Voyage à l’Isle de France, par un Officier du Roi,” part i., pages 192, 200.