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PHYSICAL GEOLOGY 



great abundance at the surface, likewise add to the deposit after 

 their death. Foraminifera are not more abundant over the deeper 

 waters than over those nearer shore, but the deposits formed 

 from their remains are not recognizable in the latter, because of the 

 large percentage of land sediments with which they are mixed. Glo- 

 bigerinaooze is seldom found in water more than one to two miles deep. 

 Below a depth of 15,000 feet the proportion of calcareous deposits 

 diminishes, owing to the increase in the percentage of carbon dioxide 

 in the water which dissolves the shells. However, many millions of 

 square miles (probably 49,520,000) of the ocean floor in temperate 

 and tropical regions are being covered by deposits of globigerina ooze 

 to-day. The chalk of England and of the western United States is 

 composed largely of the remains of Foraminifera, although the re- 

 mains of other animals are not uncommon (p. 249). 



Radiolarian Ooze. — Other unicellular animals which secrete sili- 

 ceous shells (Radiolaria) form siliceous oozes, but are found in bottoms 

 at a greater depth than the globigerina ooze, in some cases where 

 the ocean is five miles deep. 



Red Clay. — At depths greater than 15,000 feet, as has been seen, 

 the calcium carbonate of the Foraminifera is dissolved ; consequently 

 below this depth enormous areas of the ocean floor are covered with 

 extremely fine, reddish clay, composed of the insoluble portion of Fo- 

 raminifera, volcanic dust, pumice, ash, and minute meteorites. Ra- 

 diolarian ooze and red clay shade into each other in certain places, 

 the deposit being called radiolarian ooze when these organic remains 

 constitute 25 per cent, of the mass. The color of red clay is due to 

 the presence of iron oxide (Fe 2 03) formed by the oxidation of iron 

 and iron compounds. This deposit covers an area of 51,500,000 

 square miles, four fifths of which is in the Pacific Ocean, the smaller 

 area in the Atlantic being due to its lesser depth. 



The slowness with which red clay has been deposited in the past is 

 perhaps best shown by the number of sharks' teeth that are dredged 

 from the bottom. In a single haul in the south Pacific 1500 sharks' 

 teeth were brought to the surface, many of which were of extinct species. 

 It has even been suggested that, were all of the sharks alive whose 

 teeth rest upon certain areas of the ocean floor, the ocean immediately 

 above these deposits would be filled from top to bottom with living 

 flesh. The fact that meteoric dust which gathers with extreme slow- 

 ness can be detected in these deposits is a further evidence of the 

 great slowness with which the red clay accumulates. 



