206 THE OCEAN 



great ocean basins. It is characterised by the 

 abundance c" the shells of pelagic foramin- 

 ifera, and especially those belonging to the 

 genus Glohigerina. In tropical regions the 

 foraminiferous shells may be visible to the 

 naked eye, but usually the deposit appears 

 to be homogeneous and of a fawn or 

 greyish colour, sometimes forming an inco- 

 herent powder when dried. Besides foramini- 

 fera, many other calcareous remains may be 

 found in the globigerina oozes. Some of them, 

 like the pelagic molluscs and pelagic algae, 

 are derived from the surface waters; others, 

 like echinoderms, worms, molluscs, corals and 

 bryozoans, are the remains of bottom-living 

 forms. The percentage of calcium carbonate 

 in the globigerina oozes always exceeds 30, 

 and rises in the purest samples to over 90, 

 the average being usually between 60 and 70. 

 The remains of pelagic foraminifera generally 

 make up about one-half of the deposit, but 

 the amount and the species vary according to 

 latitude and depth. Within the tropics, in 

 depths of 1500 and 2000 fathoms, the percen- 

 tage of calcium carbonate due to the shells of 

 pelagic foraminifera may reach 90, and nearly 

 every known species may be represented in 

 the deposits, but on proceeding towards the 

 polar regions the percentage and the number 

 of species in the deposits from similar depths 



