328 GEOLOGY 



The occasional presence of coarse materials derived from the 

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

 accidental. ' 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. The amount which 

 icebergs might carry, if fully loaded, is far greater than the amount 

 which they do carry. 



Of the identifiable inorganic materials among the pelagic de- 

 posits, those of volcanic origin are most abundant, and among these 

 the most common is pumice, which often floats readily until it be- 

 comes waterlogged. Pieces of pumice brought up from the bottom 

 and thoroughly dried, will float for months in sea-water. The next 

 most abundant substance of volcanic origin in pelagic deposits is 

 volcanic glass. This ranges from pieces of the size of a walnut down 

 to the smallest fragments, which often serve as centers for con- 

 cretions. Lapilli (cinders) and volcanic ash also abound in parts 

 of the deep sea. The distribution of these volcanic products is 

 essentially universal, though by no means uniform. Some of them 

 are probably from submarine volcanoes. 



The deep-sea deposits contain many nodules and grains which 

 are believed to be of extra-terrestrial origin. Many of them are 

 magnetic. 1 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 other sedimentation is meager. The number of 

 meteorites which enter the atmosphere daily has been estimated 

 at from 15,000,000 to 20,000,000. 2 If, on the average, the meteor- 

 ites 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 r:itr 

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

 a layer one foot in thickness. 



1 Challenger Report, Deep Sea Deposits, p. 327. 



2 Young's Astronomy, p. 472. 



