OCR Text |
Show 180 MR. CYRIL CROSSLANO ON THE [Mar. 7, deep water, but the coarser particles of nullipore and some coral, and the shells of the foraminiferan mentioned, form beach-sand and sandstone. Between low-tide level and 3 to 5 fathoms the sand is grey from admixture of black volcanic rock. Chart o f Porto Praya. The following list of boring organisms is in the order of their importance in these islands :- ( 1 ) Sponges, yellow or red, are abundant in every kind of calcareous matter excepting the masses of Vevmetus in combination with nullipore, when the proportion of the latter is below about one half. Wherever pure nullipore is found, in tidal pools, surf-beaten rocks, or the sea-bottom, it is almost always riddled with sponge, or shows the effects of having been attacked in the past. Coral or nullipore colonies are rarely free, though the action of the sponge upon these seems to be slower. Certain species of shell are invariably attacked, e. g. those of the large species of Strombus, which is so abundant, are invariably riddled unless the mollusc is quite young. A large red Oyster is invariably attacked, |