26 A TEXTBOOK OF OCEANOGRAPHY 



for the most part derived from the remains of pelagic (as 

 distinct from demersal) forms, is, however, universally met 

 with, though more abundantly in the shallower areas. The 

 temperature is very low, being below 40 F. over the greater 

 part of the area. The conditions are uniform over very wide 

 areas. Chemical action is characteristic, and results in the 

 formation in situ of, amongst other products, glauconite, 

 phosphatic and manganese nodules, and zeolites. The change 

 from one kind of deposit to another is often very gradual, and 

 there is evidence that one deposit may overlie another, though 

 borings from the deep-sea areas are as yet scanty and of only 

 a few inches in depth. 



Terrigenous Deposits. 



The terrigenous deep-sea (hemipelagic) deposits occupy 

 15*4 per cent, of the ocean floor. The terrigenous deposits 

 of the littoral and shallow-water zones are really of the same 

 nature as those forming the terrigenous deposits of the deep 

 sea (i.e., beyond the loo-fathom line). These latter are, how- 

 ever, more uniform, homogeneous, fine-grained, and widely 

 distributed, than the former. Of the terrigenous deep-sea 

 deposits in the narrower sense Murray distinguishes three 

 classes the Blue, Red, and Green Muds. The two other 

 classes included in this group, the Volcanic Mud and Coral 

 Mud, have, as their names indicate, certain special features 

 which render their separate consideration advisable. 



Fresh water carries a much larger amount of sediment in 

 suspension than salt water ; consequently where a mixture of 

 these waters takes place there is a rapid deposit of sediment 

 on the sea bottom. This was first observed by W. H. Sidell 

 in 1837 m tne Mississipi delta,* who noticed that the rate of 

 deposit was greater than that to be expected from a mere 

 decline in the velocity of the current. Since then many 

 experiments have been made on the relative suspension 



* See Abbot and Humphreys, Report on the Missisipi, p. 876. 



