338 PROCEEDINGS OF THE 



Deep-sea deposits consist mainly of tlie organic remains of 

 three groups — Foraminifera, Eadiolaria, and Diatoms. Deep- 

 sea deposits, as opposed to " terrigenous " deposits, depend 

 for their formation on the existence of pelagic organisms having 

 both calcareous and siliceous shells. After death, and even 

 during life, these organisms slowly sink into the depths, the 

 calcareous matter of the dead shells being slowly dissolved by the 

 sea water as they sink. The organisms are of varying sizes, and 

 the carbonate of lime with which many of their shells are con- 

 structed is no doubt more readily soluble in some organisms 

 than in others. Hence the calcareous shells of the Pteropoda, 

 which, owing to their large size, more or less mask the smaller 

 shells of the Globigerinidae and other pelagic foraminifera, are 

 the most conspicuous feature of shallow-water oozes (300-700 

 fathoms), and the material is known as pteropod ooze. Before 

 the 1,000- fathom line is reached, the pteropod shells have practic- 

 ally disappeared by solution, whereas the Globigerinidae, which 

 are more resistant to solution, and also are capable of maintaining 

 existence while sinking and of continuing existence on the sea 

 bottom at great depths, come into prominence, and we call the 

 material a globigerina ooze. The long, fine spines with which 

 living globigerina are beset dissolve before they reach the bottom. 

 But the term globigerina ooze is a very elastic one. It covers 

 any deep-sea deposit in which Globigerinae predominate, and the 

 calcareous contents may vary from 90 per cent, to 20 per cent, 

 of the whole, the highest percentage being reached in tropical 

 oozes between 1,500 and 2,000 fathoms. Beyond 2,000 fathoms 

 the calcium carbonate rapidly decreases, and the ooze passes 

 gradually into red clay, the deposit which covers all the great 

 depths, and which, as a general rule, may be said to begin at 

 about 2,500 fathoms. In red clay, which is merely the residuum 

 of ooze after decalcification, the calcareous constituents are 

 always below 20 per cent, of the mass, and in great depths it is 

 practically devoid of calcareous constituents. Local conditions, 

 proximity to land, salinity and surface temperature are all 

 governing factors which influence the nature of a bottom deposit 

 at particular depths. As a general rule, solution of calcium 

 carbonate proceeds more slowly in the Atlantic than in the 

 Pacific, hence globigerina ooze is found at greater depths in the 

 Atlantic than in the Pacific ; but such oozes from great depths 



