202 THE OCEAN 



the character of the benthonic organisms living 

 in the area : off volcanic islands there are 

 volcanic gravels, sands and muds ; off coral 

 islands and reefs there are coral gravels, sands 

 and muds ; off continental coasts there are 

 usually quartz gravels, quartz sands and marls. 

 In depths of about 100 fathoms (600 feet) 

 the limit of wave action and of strong trans- 

 porting currents is reached, and all the minute 

 detrital matters come permanently to rest 

 on the bottom at what has been called the 

 mud-line. Beyond this depth the deposits 

 become much more uniform in their physical 

 characters and composition, although they 

 still derive their general characteristics from 

 the adjoining lands and coasts, but may fre- 

 quently present a considerable admixture of 

 the remains of pelagic organisms. All deposits 

 laid down in water deeper than 100 fathoms 

 are called deep-sea deposits, and to these we 

 shall here limit our remarks. 



Terrigenous Deposits. 



1. Blue Mud. — This type is the one most 

 frequently met with in the deeper waters 

 surrounding continental land and in all 

 enclosed and partially enclosed seas. The 

 deposit is so called because it is usually of 

 a blue or slate colour, with a thin upper 

 red or brown layer where it has been in 



