174 MARINE DEPOSITS 



but some indication is given by the raised terrace of modern lime- 

 stone which occurs in northern Yucatan. In this are caverns 

 which descend through more than 400 feet of such rock (without 

 reaching the bottom), all of which is formed from the hard parts of 

 the same species of animals as still abound in the neighbouring seas. 



Chemical Deposits. — It is not known just how important a part 

 is played by chemical precipitation in the formation of marine 

 deposits, but probably a greater one than has been generally sup- 

 posed. Rivers which bring in quantities of CaC0 3 in solution 

 may so overload the sea with this substance (for sea- water will 

 dissolve little of it) that more or less is precipitated in the neigh- 

 bourhood of the land. On the coast of Asia Minor, for example, 

 are large areas of sandstone and conglomerate, formed within recent 

 times by the precipitation of CaC0 3 in masses of sand and gravel, 

 binding them into hard rock. Similar examples are known else- 

 where. There is also some reason to believe that the decay of 

 marine animals evolves sufficient carbonate of ammonia to con- 

 vert the sulphate of lime into the carbonate by double decompo- 

 sition, and to precipitate the latter in some quantity. 



3. Deep-sea Deposits. — The 100-fathom line is by Murray 

 and Renard regarded as the boundary between shallow and deep 

 water, for it generally marks the edge of the continental shelf, 

 from which the bottom rises very gently to the land, but slopes 

 abruptly down to the oceanic depression. The great bulk of 

 the material derived from the waste of the land is thrown down 

 upon the continental shelf, within the 100-fathom line, but the 

 finer particles are carried farther out and subside in deeper and 

 quieter water. A considerable quantity of the finest sedimentary 

 particles remains long suspended in sea-water, especially in the 

 cold water of the polar seas. On the continental slopes, extend- 

 ing from the 100-fathom line to the bottom of the great oceanic 

 abysses, are laid down most of the very fine materials derived from 

 the land, which are grouped together under the somewhat indefi- 

 nite term, mud. 



a. Terrigenous Deposits are composed of materials chiefly de- 

 rived from the shore, and occur in the less profound depths. 



