IV DEPTHS AND DEPOSITS OF THE OCEAN 163 



in the surface waters, and where there is abundant secretion of 



silica by the plankton 

 organisms. Over wide 

 areas in very deep water, 

 however, neither cal- 

 careous nor siliceous 

 remains predominate ; 

 the basis of the deposit 

 then becomes Red clay, 

 consisting of clayey 

 matter derived from the 

 decomposition of vol- 

 canic materials ; quartz 

 particles, so abundant 

 in terrigenous deposits, 

 are rare or absent. 



The pelagic deposits 

 are subdivided into the 

 following types, viz. : — 



Pteropod Ooze. — In Pteropod ooze. 

 the shallower waters, 

 oceanic ridges and cones, 



Fig. 136. — Pteropod Ooze. 



Valdivia" Station 208, Indian Ocean, lat. 6° 54' N. 



long. 93° 28'. 8 E., 162 fathonis (magnified). 



on 



usually far from continental land 

 especially within coral 

 reef regions where 

 warm water with small 

 annual range occupies 

 the surface, almost 

 every surface organism 

 which secretes a hard 

 shell or skeleton is 

 represented in the de- 

 posit, the dead shells of 

 pteropods and hetero- 

 pods being character- 

 istic, and the deposit is 

 hence called Pteropod 

 ooze (see Fig. 136). 

 About 35 species of 

 pteropods and 32 

 species of heteropods, 

 as well as pelagic gas- 

 teropods (see pp. 172- 

 173), are known to live in the surface waters of the tropics, and 



Fig. 137. -Gi.onicERiNA Ooze. 

 Valdivia" Station 45, Atlantic, lat. 2° 56'.4 N., 

 long. 11° 40'. 5 W., 2728 fathoms (magnified). 



