MARINE DEPOSITS 195 



In the case of rivers the materials are either 

 in solution or suspension, and the particles 

 in suspension are for the most part deposited 

 on the sea-floor whenever the salt and fresh 

 water mix. It is well known that fine clayey- 

 matter in fresh water is at once precipitated 

 upon the addition of a little salt. The 

 detrital matters carried to the ocean by rivers 

 are thus laid down near shore, the sands and 

 gravels coming to rest in shallow water, and 

 the finer clayey particles being deposited in 

 deeper water. 



The winds often carry the dust of deserts 

 and of volcanic eruptions very great distances, 

 and these can be detected in the deposits of 

 deep water. Another volcanic product re- 

 quires special mention, viz., pumice. This 

 areolar substance, when it reaches the ocean 

 either from rivers or volcanic outbursts, floats 

 for a long time on the surface. The separate 

 blocks are knocked against one another by 

 the waves ; small particles are broken off and 

 fall to the bottom, and thus pumice is dis- 

 seminated all over the sea-floor. The floating 

 blocks are sometimes covered with cirripeds 

 and other marine animals, but eventually 

 they become water-logged and sink to the 

 bottom, where they are ultimately decom- 

 posed into clay. Very many samples of 

 these pumice stones have been dredged from 



