180 



MK. CYRIL CROSSLAND ON THE 



[Mar. 7, 



deep water, but the coaiser pai'ticles 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. 



Text-%. 25. 



'I'j'S 



Chart of 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 mattei- excepting the masses of Vermetus in combination 

 with nullipore, when the proportion of the lattei- is below about 

 one half. Wherever pure nullipore is found, in tidal pools, sui-f- 

 beaten rocks, or the sea-bottom, it is almost always riddled with 

 sponge, or shows the efi'ects 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, 



