406 HYDROGRAPHICAL SURVEYING [chap, xviii. 



laria), intermixed occasionally with a few globigerina shells. 

 When so intermixed it effervesces slightly with dilute hydro- 

 chloric acid, but when pure it does not effervesce. The 

 skeletons of these radiolaria are beautiful objects under the 

 microscope. This deposit has hitherto been found only in 

 the Pacific and Indian Oceans, at depths exceeding 2,300 

 fathoms. 



6. Red, Grey, or Chocolate Clays are widely distributed over ~ 

 the floor of the ocean at depths exceeding 2,100 fathoms. The 

 red colour is due to the presence of oxide of iron, and the 

 chocolate colour to peroxide of manganese. Pumice stone, 

 manganese nodules, sharks' teeth, and the ear-bones of whales, 

 have frequently been found embedded in the clays, which do 

 not effervesce when treated with acid, unless, which is occa- 

 sionally the case, they have a slight intermixture of globigerina, 

 ooze. 



General Remarks. — All oceanic deposits contain small black, 

 red, and yellow magnetic particles, but these are not abundant 

 in the clays. Should the sounding tube come up empty, as it 

 does occasionally, though rarely, it should be carefully ex- 

 amined on the outside for black-brown streaks, as these 

 indicate the presence of oxide of manganese at the bottom in 

 the form of nodules or stones too large for the tube to bring up. 



As it is important to describe the substances brought up in 

 the sounding tube by terms readily recognisable, and as the 

 foregoing were adopted after much consideration by the 

 scientific staff of the Challenger, it is recommended that sur- 

 veyors employed in deep-sea sounding should endeavour so 

 far as is practicable to follow this nomenclatiire. 



SUBMARINE SENTRY. 



The use of the submarine sentry has already been re- 

 ferred to on p. 216, in cormection with searching for vigias. 

 It supphes an automatic warning of the approach of a ship 

 to shallow water, and a description of the apparatus is here 

 appended. 



By means of a single stout wire, the sinker, an inverted kite, 

 called the " sentry," can be towed steadily for any length of 



