THE OCEAN AND ITS WORK 241 



Subsidence Necessary for Great Accumulations. — Sediments do 

 not accumulate to a great thickness unless the sea bottom upon 

 which they are being laid down is subsiding. This has not been 

 an uncommon condition in the past, as is shown by the occurrence 

 of stratified rock four and even more miles in thickness. Many 

 of these deposits are of shallow-water or of continental origin, as the 

 ripple and rill marks, the coarseness of some of the ingredients, and 

 the fossils show. 



Deep-sea Deposits 



Deep-sea deposits cover about three fifths of the sea bottom, and 

 are found beyond the limit of the sediments derived from the land. 



Blue Mud. — Since an ocean depth greater than 600 feet is usually 

 more than 10 miles from the shores, only those sediments which are so 

 fine that they can be carried in suspension for a long distance are found 

 in such situations. The sediments are usually of a bluish gray color 

 and are classed roughly as blue muds. The bluish gray color is due 

 to the fact that the contained organic matter prevents the oxidation 

 of the iron in the deposits. They cover an area of approximately 

 15,000,000 square miles of the ocean bottom, or five times the extent 

 of the United States. They surround all coasts, beyond the shoal 

 deposits, and cover the deeper parts of such inland seas as the 

 Mediterranean. The depth of water in which they occur varies from 

 about 750 feet to 16,800 feet. 



Globigerina Ooze. — This ooze is a deposit consisting of 30 to 90 

 per cent, of the shells of Foraminifera (Fig. 234), of which the most 

 abundant genus is Globigerina. These 

 unicellular animals seldom attain a size 

 greater than that of a pinhead, and 

 secrete a shell (test) of calcium car- 

 bonate. They are extremely simple in 

 structure, but the shells as seen through 

 a microscope are very beautiful. They 

 live in countless millions in the surface 

 and subsurface waters of the ocean and Fig 234. - Globigetina ooze, 

 . greatly magnified. (After 



upon their death rain down on the sea Shimer.) 



floor. It is not to be understood that 



these organisms are the only ones whose remains constitute this 



widespread deposit. Other small forms of life, which also live in 



