THE WORK OF THE OCEAN. 381 



situations. In general, these deposits are analogous to certain shales, 

 marls, etc., found within the continents. 



Though coarse materials derived from the land are occasionally 

 found in the deep-sea deposits, their presence must be looked upon as 

 in some sense accidental. Occasional pebbles, or even bowlders, are 

 carried out into the ocean entangled in the roots of floating trees. 

 Within limits, too, icebergs have carried out land debris, though it is 

 probable that transportation by this means has been exaggerated. 

 The amount which icebergs might carry, if fully loaded, is far greater 

 than the amount which they do carry. 



Of the identifiable inorganic materials in the deep sea, the most 

 abundant are of volcanic origin, and among these the most common is 

 pumice, which is frequently so light that it floats readily until it becomes 

 water-logged. Pieces of pumice brought up by the Challenger and 

 thoroughly dried were found to float for months in sea-water before 

 settling even through the depth of water contained in the vessel in which 

 the experiment was performed.* 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 concretions. Lapilli (cindei;s) and 

 volcanic ash also are abundant in parts of tjiejdeep. sea. The disti^i^ 

 bution of these volcanic products is essentially uniyersal, though by no 

 means uniform. Some of them are probably from submarine volcanoes. 



The study of the deep sea deposits has revealed the presence of 

 many nodules and grains whijh are believed to be, ipf extra-terrestrial 

 origin. Many of them are magnetic.^ The dust of countless meteors 

 which enter the atmosphere dily settles on land and sea ahke, and 

 enters into the sediment of 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 more meager. The num- 

 ber of meteorites which enter the atmosphere daily has been esti- 

 mated at from 15,000,000 .o 20,000,000.' 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, oix 



* Murray, loc. cit., p. 295. 



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



•Young's Astronomy, p. 472. 



