210 THE SEA FLOOR 



i. Materials of all kinds borne to the ocean by rivers, 

 winds, and floating ice, as well as what is torn from the 

 coasts by waves, currents, and other agencies ; these 

 are termed " terrigenous " or " earth born." 



2. Materials derived from volcanic eruptions, both 

 submarine and subaerial. Volcanic dust is transported 

 immense distances by the wind. Pumice of various 

 types is worthy of special notice, because its areolar 

 spongy structure, containing cavities filled with gas, 

 admits of very wide distribution, by drifting on the 

 surface of the ocean. 



3. Materials of extra-terrestrial origin, which have 

 fallen to the earth from interstellar space, such as 

 cosmic spherules of iron and nickel, and chondrites 

 with lamellar structure. 



4. Materials of organic origin may be carried to the 

 ocean by rivers and winds, but the chief source is the 

 hard parts — the shells and skeletons — of organisms 

 which live in the surface, intermediate, or bottom 

 waters of the ocean. These may be composed of lime 

 (calcium carbonate), such as calcareous Algae, Fora- 

 minifera, Corals, Alcyonarian and Tunicate spicules, 

 worm tubes, Ostracod and Cirriped shells, Echinoid 

 spines, shells of Molluscs, Polyzoa, bones of fishes and 

 whales, etc. ; or they may be composed of flinty 

 (siliceous) spicules and frustules, such as Diatoms, 

 Radiolarians, and Sponge spicules. In these organic 

 remains it is important to distinguish between those 

 which belong to floating or Plankton organisms, and 

 those which belong to bottom-living or Benthos 

 organisms. Generally speaking, it has been found that 

 the remains of Plankton organisms predominate in the 

 deposits of deep water far from land, and the remains 

 of bottom organisms predominate in the deposits of 

 shallow waters near shore. 



