CHAP, xvm.] NATURE OF OCEAN BOTTOM 405 



the dredge. Ease out the hawser to about three times the 

 depth of water, and let the ship drift for about 20 or 30 minutes. 

 The dredge can be hoisted in by a burton from the mizzen 

 gaff. 



NATURE OF OCEAN BOTTOM. 



The deposits on the floor of the ocean may be classed under 

 the foUowmg heads : 



1. Shore Deposits. — Within a distance of about 200 miles 

 from land the deposits partake of the nature of the coast, thus : 

 Around volcanic islands the deposits are gi'ey or black in colour, 

 and consist chiefly of pumice, scoriae, and volcanic sand. 

 Around coral islands the deposits are white, and consist of the 

 detritus of the neighbouring reefs ; whilst in the vicinity of 

 land which is not volcanic or coral the deposits are usually 

 blue or green muds, and consist chiefly of the detritus of rivers 

 and washings of the coast. These latter contain frequently 

 some surface shells and diatoms. The green muds are especially 

 interesting, as they generally contain some beautiful internal 

 casts of carbonate of lime organisms in glauconite. 



2. Globigerina Ooze is widely distributed over the bed of the 

 ocean. It is of a white or light brown colour and sticky nature, 

 and consists chiefly of minute globular shells of carbonate of 

 lime, called globigerina. It is easily known by its appearance 

 under the microscope, and from the fact of its effervescing 

 strongly when treated with dilute hydrochloric acid. It has 

 occasionally been found at a depth of 2,800 fathoms, but is 

 usually purest at 2,000 fathoms. 



3. Pteropod Ooze is somewhat similar to globigerina ooze, 

 but consists of the sheUs of animals (Pteropods), which can be 

 seen \vithout artificial aid. It also effervesces when treated 

 with dilute acid, but generally will be found at depths under 

 1,200 fathoms when little or no land debris exists. 



4. Diatom Ooze is of a white or rose colour, and is chiefly 

 composed of the silicious casts of minute plants (Diatoms). 

 It effervesces but slightly, if at all, when treated with acid, 

 and has hitherto been found only in the Antarctic Ocean. 



5. Radiolarian Ooze may be white or brown in colour, and 

 is composed chiefly of the skeletons of minute animals (Radio- 



