44 A TEXTBOOK OF OCEANOGRAPHY 



The fine washings consist essentially of hydrated silicate 

 of alumina, 



Red clay lias not been found in less depths than 2,200 

 fathoms. Radioactive substances are found more abundantly 

 in red clay than in any other marine deposit, or in any 

 continental rocks. 



Red clay is the most widely distributed oceanic deposit. 

 It occupies the deepest portions of the great océans, except in 

 Polar régions, extending from 50° N. to 50° S. in the Pacific, 

 where it is the main deposit, and between 40° N. and 40° S. 

 in the Atlantic. 



Stratification in Marine Deposits. 



Samples of marine deposits are only known to a depth of 

 I or 2 feet, so tliat our knowledge of their stratification is slight. 



The tubes attached to sounding leads do not bring up 

 portions of the bottom deposit of more than a foot in depth, 

 except in rare instances. In thèse samples there is sometimes 

 clear évidence of stratification. Phillipi, who examined the 

 bottom samples collected by the German South Polar investi- 

 gation steamer Gauss, believes that stratification is the rule in 

 bottom deposits. 



Generally speaking, there is a différence in colour and 

 chemical composition between différent layers of the same 

 deposit — e.g., blue mud is stiff and blue in colour in its lower 

 portion, thinner and reddish-brown in its upper layers. This 

 is probably due to the ferrie oxide or ferrie hydrate changing 

 into the sulphide or ferrous oxide in the deeper layers. There 

 are examples of — 



(1) Globigerina ooze above red clay, found by the 

 Challenger expédition in the South Pacific, pointing to an 

 élévation of the océan floor. 



(2) Red clay above globigerina ooze, found by the 

 Challenger also in the South Pacific, pointing to a subsidence 

 of the océan floor. 



(3) Globigerina ooze over blue mud. 



