38 AGRICULTURE. 



These deposits were not made out in the open 

 sea, but near the shore in shallow wrter. Their 

 thickness is accounted for by the iheory that 

 the ocean bottom was sinking gradually, and 

 fresh deposits were made above the preceding 

 ones. 



In the open sea are found the deposits of 

 very fine particles carried out by the rivers — on 

 the continental slopes from the one hundred 

 fathom line to the oceanic abysses — and they 

 are known under the indefinite term of "mud." 

 There are also volcanic deposits and great accu- 

 mulations of organic remains. Every animal in 

 the sea which has a shell or hard skeleton helps 

 to make these deposits, but by far the greater 

 part of them is made up of the shells of minute 

 organisms* which live near the surface. The 

 diatom ooze is composed of the siliceous re- 

 mains of microscopic plants. 



(6) Frost. — Frozen water has done a great 

 share of the work in this process of mantling 

 the earth with loose material. Some rocks are 

 more porous than others, though those appar- 

 ently solid will, upon examination, be found to 

 be crossed by joints which divide them into 

 blocks. These are filled with minute crevices 

 and pores, through which the water percolates 

 even to the very center. Water, upon passing 



* Jordan and Kellogg's Animal Life, p. iS. Scott's Geology, 

 pp. 176-180. 



