GEOLOGIC TIME. 



653 



careous oozes and muds, we have a vast formation greatly exceed- 

 ing in bulk and extent the coral reefs of tropical seas ; they are 

 most widely distributed in equatorial regions, but some patches 

 of Globigerina ooze are to be found even within the Arctic circle 

 in the course of the gulf stream." 1 



The percentage of carbonate of lime contained in deposits 

 accumulating at different depths, as obtained from 231 samples 

 collected by the Challenger, is shown in the following tabulation : 



14 cases under 500 fathoms, m. p. c. 86.04 



7 " " 500 to 1000 " " 66.86 

 24 " " 1000 to 1500 " '* 70.87 

 42 " " 1500 to 2000 " " 69.55 

 68 " " 2000 to 2500 " " 46.73 

 65 " " 2500 to 3000 " " 17.36 



8 " " 3000 to 3500 " " 0.88 

 2 " " 3500 to 4000 " " 0.00 

 1 " " 4000 " " trace. 



The fourteen samples under 500 fathoms are chiefly coral 

 muds and sands, and the seven samples from 500 to 1000 fath- 

 oms contain a considerable quantity of mineral particles from 

 continents or volcanic islands. In all the depths greater than 

 1000 fathoms the carbonate of lime is mostly derived from the 

 shells of pelagic organisms that have fallen from the surface 

 waters, and it will be noticed that these wholly disappear from 

 the greater depths. 2 



By a series of experiments Messrs. Murray and Irvine found : 

 "That although sea water under certain conditions may take up 

 a considerable quantity of carbonate of lime in solution, yet it is 

 unable permanently to retain in solution more than is usually 

 found to be present in sea water, and it is owing to this that the 

 amount of carbonate of lime is so constantly low. The reaction 

 between organic matter and the sulphates present in sea water 

 (to which we have referred) tends also to keep the amount of 

 carbonate of lime in solution at about one-half (0.12 grms.) of 

 what it might contain (0.28 grms. per litre). This peculiarity 

 of sea water, in taking up a large amount of amorphous carbon- 



I Loc. cit., pp. 82-83. 

 2 Loc. cit., p. 84. 



