ABYSSAL DEPOSITS 679 



heaps of small grains or lumps. They range in depth to over 8,000 

 meters, being especially alnnidant in the Pacific. They are, how- 

 ever, also found in shallow water, as in the case of the deposits 

 formed in Loch Fyne in Scotland. ( Buchanan. ) The mineral is 

 generally the hydrous oxide, and, according to ^^1 array, is a product 

 of disintegration of the volcanic rocks found so abundantly in these 

 deeper waters. The manganese and iron derived from the vol- 

 canics are at first in the form of carbonate, after which they become 

 altered to the oxide. The concretions commonly contain fragments 

 of pumice or lapilli or a bone or other organic structure as a nucleus. 

 (See also Chapter IX.) Besides the manganese concretions there 

 are concretions of other minerals, of which barium is one. 



Cosmic Deposits in the Deep Sea. 



Fine particles with a metallic interior often magnetic are found 

 in the deeper water deposits. These have been interpreted as cosmic 

 dust, the product of meteoric showers. Chamberlin and Salisbury 

 (16: j8i-2) state that the number of meteorites which enter the 

 atmosphere daily has been estimated at from 15,000,000 to 20,- 

 000,000, and that if on the average each weighs 10 grains (a high 

 estimate), the total amount of extra-terrestrial matter reaching the 

 earth yearly would be 5,000 to 7,000 tons, of which about three- 

 fourths on the average would fall into the sea. At this rate it would 

 take some fifty billion years to cover the sea bottom with a layer 

 one foot in thickness. 



Submarine Volcanic Deposits. 



Deposits formed on the floor of the ocean by submarine volcanic 

 eruptions probably constitute an important part of the deep-sea 

 deposits. Not all of the widely distributed volcanic material, how- 

 ever, found in the deep sea is the product of submarine volcanic 

 eruptions ; a large part of the pumice, lapilli and volcanic glass and 

 dust is derived from terrestrial volcanoes, and is carried to the deep 

 sea by flotation on the surface of the ocean, or as wind-borne 

 dust, or as both. The Hawaiian Islands represent the result of 

 prolonged submarine volcanic eruptions, and the accumulation of 

 the material in the vicinity, until they rose to the present height of 

 the sea-level. For the general subject of submarine eruptions, sec 

 Thoulet (94). (See also Chapter XXII.) 



