GROWTH OF GEOLOGY. 281 



Renard a most careful study has been made of the 

 marine deposits on the ocean-floor. These are con- 

 veniently divided into two sets — (1) the terrigenous 

 deposits, for the most part consisting of the dis- 

 integrated particles of the adjacent emerged land, 

 and of great interest as illustrating accumulations 

 analogous to those which formed many of the strati- 

 fied rocks; and (2) pelagic deposits, which begin at 

 an average of about 200 miles from the continental 

 coast-lines, and are mainly composed of the shells 

 of pelagic organisms (Molluscs, Foraminifera, Ra- 

 diolaria. Diatoms, etc.), besides inorganic particles 

 of volcanic or cosmic origin. The " Red Clay," 

 which covers nearly half of the sea-floor, and all the 

 deeper parts, is probably due to the chemical altera- 

 tion of organic and inorganic remains during a pro- 

 longed period of slow accumulation. Sir John 

 Murray argues that the number of sharks^ teeth, of 

 earbones and other bones of whales, and of cosmic 

 spherules in a deposit may be taken as a measure 

 of the rate of deposition. These bodies are most 

 abundant in the Red Clay, probably because few 

 other substances reach the great depths to cover 

 them up. " One haul of a small trawl in the Central 

 Pacific brought to the siirface on one occasion, from 

 a depth of about 2J miles, many hundreds of man- 

 ganese nodules, along with 1500 sharks' teeth, over 

 50 fragments of earbones and other bones of 

 whales." 



It may seem to the careless both dull and unprofit- 

 able to map out with care the sediments which are 

 now forming on the floor of the ocean, but the im- 

 portance of these maps to the geologist is immense. 

 For it is from them that we can argue back to the 

 history of the sedimentary part of the earth's crust, 



