184 MR. CYRIL CROSSLAND ON THE [Mar. 7, 



over with the usual incrustation above described. Of the seaward 

 jjrojecting 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 hei-e 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 Vermetits-tx^hea 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 lidges 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-\}\^}o%^ 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 sti-ata being neai^y horizontal or with a slight but distinct 

 dip landwards. The surface is cut into hollows and sharp ridges 

 in the way characteristic of " Ooral-rag." In composition the 

 rock is practically completely calcareous, consisting of an aggluti- 

 nation of foraminifera (the lai'ge species mentioned as forming 

 the coarse sand of the bay) and more or less finelj^ broken shells. 

 It is noteworthy 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 2'>i'esent day, are frequently met 

 with. This rock is bored by the Lamellibranch Lithopliagus and 

 the Echinid Echinometra suhangularis, but not by Sponge or 

 Polychfeta, &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 no 

 rivers in St. Vincent (where the rainfall of the past three years 

 has amounted to just thi-ee 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 per- 

 colates down their beds, and even flows over their surfaces after 

 very heavy rain. This water takes up lime in solution from the 



