212 THE SEA FLOOR 



boulders and varied rock-fragments have been fre- 

 quently dredged up hundreds of miles from land. It 

 is known that seals and penguins may transport rock- 

 fragments in their stomachs to great distances. For 

 these, among other reasons, the size of the particles is 

 of little use for the purpose of the classification of 

 marine deposits. 



While the detrital matter from continents and islands, 

 and the shells and skeletons of bottom-living organisms, 

 prevail in the deposits near shore and in shallow water, 

 the remains of pelagic or Plankton organisms, and 

 chemical or secondary products, prevail in all the deep 

 water deposits far from land. It is a remarkable fact, 

 how r ever, that in areas where the depth is 3,000 fathoms 

 or deeper, hardly a trace of calcareous pelagic shells, 

 such as those of Foraminifera and Pteropods, is to be 

 found in the deposit, although these may be met with 

 in great abundance in an adjacent area where the depth 

 is only 1,500 or 1,000 fathoms. In like manner the 

 skeletons of Radiolaria and the frustules of diatoms are 

 not nearly so abundant in some deposits as would be 

 expected from the numbers captured in the surface- 

 waters of the region. Both calcareous and siliceous 

 remains are dissolved by sea water, and when they are 

 not specially abundant in the surface waters, they may 

 be very rare in, or wholly absent from, the deposits at 

 the bottom. At other times these remains of pelagic 

 organisms appear to be almost entirely masked by the 

 terrigenous materials, or other organic remains, forming 

 the deposit. 



From the above considerations it is evident that the 

 materials which go to form marine deposits vary in 

 nature, abundance, and size, according to the structure 

 of the nearest land, the character of the submarine 

 volcanic eruptions, the distance from the shore line, 



