IV DEPTHS AND DEPOSITS OF THE OCEAN 157 



ocean-floor. Where basic volcanic rocks are in process of 

 decomposition, manganese nodules may be relatively abundant 



in shallow water, and they are never 

 numerous in Globigerina oozes, ex- 

 cept where volcanic material is 

 present in some abundance in the 

 deposit. 



Sulphate of barium has been Barium. 

 found to be present in most marine 

 deposits and in manganese nodules 

 in small quantities ; in terrigenous 

 deposits up to about o. i per cent, in 

 manganese nodules slightly more, 

 and in Red clays up to about i per 

 cent. Small round nodules have 

 been trawled off Colombo, in 675 

 fathoms, containing 75 per cent of 

 barium sulphate. 



-Glauconite occurs in the terri- Giauconite. 

 genous deposits typically in the form 

 of minute rounded grains of a green- 

 ish colour, usually associated with greenish or brownish casts of 

 calcareous organisms (foraminifera, etc.) ; in fact, the rounded 



Fig. 127. — Tooth of Oxvrjj/xa 



TRIGODON. 



"Challenger" Station 276, Tropical 



Pacific, 2350 fathoms. 



Fig. 128. — Petrous and Tympanic Bone 

 of ziphws cavirostris. 



"Challenger" Station 286, South Pacific, 

 2335 fathoms. 



Fig. 129. — Section of a Mangan- 

 ese Nodule, showing a Tym- 

 panic Bone of Mesoplodon in 

 the Centre. 

 "Challenger" Station 160, Southern 

 Ocean, 2600 fathoms. 



trace 



green grains are supposed to be casts which have lost all 



of the enveloping calcareous chambers. The individual grains 



of glauconite do not exceed one millimetre in diameter, though 



