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Show 184 MR. CYRIL CROSSLAND ON THE [Mar.. Tr over with the usual incrustation above described. Of the seawar d projecting masses the raised part is in some cases entirely composed of organic matter, of others the corresponding rock is sandstone thinly coated with nullipore. It is here that the mode of growth of the Vermetus is best seen. Under the influence of the sea calcareous rocks usually become hollowed into a series of concavities separated by sharp ridges and pinnacles. On the seaward side of this reef these ridges are occupied by Vermetus-tubes alone. The coils of shell grow out horizontally from the top of the ridge, on either side, forming broad cake-like masses which cover over the depressions between the adjacent ridges as though one had laid a flat stone across. In the same way, but on a smaller scale, are doubtless formed those cavities in the incrustations of the lava rocks which are the habitat of an important constituent of the fauna. As one passes to the actual edge of the reef these colonies of Vermetus become combined with nullipore, the proportion of which rises until at the extreme edge the compound rock is half or more than half nullipore. Rock-boring organisms are here comparatively rare, and Sponges and Polychseta of this habit are entirely absent! The other borers are common enough, but the Vermetus-tubes are apparently too hard and the intervening layers of nullipore too narrow for these. The rock beneath is a sandstone of very variable fineness and hardness, usually harder at its seaward edge, and softest at the top of the beach-when it occurs there. It is distinctly stratified, the strata being nearly horizontal or with a slight but distinct dip landwards. The surface is cut into hollows and sharp ridges in the way characteristic of " Coral-rag." In composition the rock is practically completely calcareous, consisting of an agglutination of foraminifera (the large species mentioned as forming the coarse sand of the bay) and more or less finely broken shells. It is notewoi'thy that the sand of the beach consists of exactly the same materials and in both is found a small quantity of dark grey sand of volcanic origin. Rolled black pebbles are included, and sometimes the rock is almost a conglomerate of such, and shells, apparently those of the present day, are frequently met with. This rock is bored by the Lamellibranch Lithophagus and the Echinid Ecliinometra subangularis, but not by Sponge or Polychpeta, &c. The formation of this rock has taken place in the same way as the sandstone-reefs off the river-mouths of Brazil. There are 110 rivers in St. Vincent (where the rainfall of the past three years has amounted to just three inches), but these reefs are situated at the mouths of two flat valleys which slope gently up to the mountains of the centre of the island, and are separated by the hill on the shore referred to above. Water continually percolates down their beds, and even flows over their surfaces after very heavy rain. This water takes up lime in solution from the |