8o 



MARINE DEPOSITS 



FIG. 75. — Diatom ooze. X 150. 

 (Agassiz after Murray and Renard.) 



diatoms form considerable accumulations in lakes and ponds, and 

 they also nourish abundantly in brackish water and in the sea. 



Diatoms are found in many marine 

 deposits, but in relatively small quan- 

 tities. In the Antarctic Ocean, how- 

 ever, is an immense belt of ooze, 

 believed to cover 10,880,000 square 

 miles and extending around the 

 globe, which is largely made up of 

 their frustules. Besides the great 

 Antarctic zone, an area of some 

 40,000 square miles is known in the 

 North Pacific. The diatom ooze 

 entirely resembles the fresh-water 

 deposit, but may be distinguished 

 by the presence of foraminiferal and radiolarian shells and tests. 

 The depths at which this ooze is found are from 600 to 2000 

 fathoms. 



(5) Red Clay. — The profoundest abysses of the ocean, far 

 from any land, are covered with a deposit of red clay, which, 

 though varying much in composition and colour, is yet of a 

 quite uniform character. In these vast depths the foraminiferal 

 shells are almost all dissolved by the carbonated sea-water, but 

 some CaC0 3 is very generally present, averaging about 6%, 

 and diminishing in quantity as the depth increases. In the less 

 profound abysses the red clay passes gradually into the forami- 

 niferal oozes, the number of shells increasing until the ooze- 

 like character is attained. The clay is derived from the disin- 

 tegration and decay of volcanic substances, especially pumice, 

 which floats upon water, often for months, and drifts long dis- 

 tances in the ocean currents. The greater part of these volcanic 

 materials is believed to be derived from terrestrial volcanoes, but 

 the submarine vents doubtless contribute largely; particles of 

 undecomposed volcanic minerals and glasses are also common. 

 In some regions the clay is coloured chocolate brown by the oxide 

 of manganese, and many separate nodules of this substance are 



