MARINE DEPOSITS 219 



being saturated, the water in contact with a 

 calcareous deposit must be very near satura- 

 tion-point and has therefore less solvent power. 

 Solution is, however, apparently more active 

 beyond 2000 fathoms, which may be due to the 

 lower temperature, to the increased pressure, 

 and to the greater abundance of carbonic acid 

 in the water. In areas occupied by massive 

 warm surface currents, where lime-secreting 

 organisms are more abundant than elsewhere, 

 the shells accumulate on the bottom at greater 

 depths than usual, as is also the case in areas 

 where warm and cold currents meet at the 

 surface, where there is reason to believe that 

 organisms are killed in greater numbers 

 than elsewhere by the sudden changes of 

 temperature. 



In those parts of the ocean-floor covered by 

 chocolate-coloured red clays, characterised 

 by the abundance of manganese in nodules 

 and grains, as in the Central Pacific, it seems 

 as though the calcareous shells were removed 

 from the deposit at less depths than usual, 

 possibly by some hypogene action. Thus 

 red clays are found in the Pacific in depths at 

 which globigerina oozes occur in the Atlantic. 



The amount of calcium carbonate secreted 

 by organisms from sea-water must be enor- 

 mous, and since only a comparatively small 

 quantity is present in solution, lime-secreting 



