ig8 WORK OF THE OCEAN 



the finest products of land decay, carried out by movements of 

 water and by winds. They are not commonly recognized in the 

 dredgings more than 200 miles from shore, but opposite the mouths 

 of great rivers they extend much farther 1,000 miles in the case 

 of the Amazon. They are especially abundant on the slopes of the 

 continental shelves, where the blue, green, and red muds are asso- 

 ciated with volcanic and coral muds. The color of these various 

 muds depends in part at least on the changes they have undergone 

 since their deposition. These deposits are analogous, in a general 

 way, to certain shales, marls, etc., found on the continents. 



The occasional presence of coarse materials from the land in 

 the deep-sea deposits must be looked upon as in some sense acci- 

 dental. Pebbles, or even bowlders, entangled in the roots of 

 floating trees, may be carried out into the ocean, and icebergs carry 

 out bowlders and smaller fragments of rock. Of the identifiable 

 inorganic materials in the pelagic deposits, those of volcanic origin 

 are most abundant. Their distribution is essentially universal, 

 though not uniform. Some of them are probably from submarine 

 volcanoes. 



Deep-sea deposits contain many nodules and grains believed 

 to be of extra-terrestrial origin. The dust of the countless meteors 

 which enter the atmosphere daily settles on land and sea alike, and 

 must enter into the sediment at the bottom of the latter. It is 

 probably no more abundant in deep water than in shallow, but it is 

 relatively more important, since there is little other sediment. The 

 number of meteorites which enter the atmosphere daily has been 

 estimated at from 15,000,000 to 20,000,000. 1 If, on the average, 

 they weigh ten grains each, probably a rather high estimate, the 

 total amount of extra-terrestrial matter reaching the earth yearly 

 would be 5,000 to 7,000 tons, and something like three-fourths of 

 this must, on the average, fall into the sea. But even at this rate 

 it would take some fifty billion years to cover the sea-bottom with 

 a layer one foot in thickness. 



Organic constituents of pelagic deposits. With increasing dis- 

 tance from shore, and especially with increasing depth of water, 

 sediments derived from pelagic life increase in relative importance. 

 Some pelagic animals and plants secrete lime carbonate, while 

 diatoms and radiolarians secrete silica. When the organisms die, 



1 Young's Astronomy, p. 472. It is now believed that these figures are too 

 small. 



