196 WORK OF THE OCEAN 



The magnesium salts are among the last to be thrown down as 

 sea-water is evaporated, and they most commonly take the form 

 of sulphates and chlorides. The magnesium salts are among the 

 last to be precipitated, not only because they are readily soluble, 

 but because their quantity is small; yet in the original rock from 

 which the sea-salts came, there is at least as much magnesium as 

 sodium, while in the sea there is about five times as much sodium 

 as magnesium. Just what becomes of all the magnesium brought to 

 the sea-water is not well understood. In the older marine strata, 

 dolomite, composed partly or wholly of the double carbonate of 

 lime and magnesia (CaMg)COs, abounds. This appears to have 

 been formed by a gradual substitution of magnesium for calcium 

 in calcium carbonate, but just b^ow and wjien and why the substi- 

 tution was effected is not fully known. One view is that dolo- 

 mite was formed chiefly in basins not freely connected with the sea. 



The plants and animals of the sea secrete notable quantities of 

 silica, but deposits of this sort are relatively more important in the 

 deep sea, and will be mentioned in that connection. 



Something concerning the origin of limestone has already been 

 given in the preceding paragraphs; but because of the importance 

 of this rock, it may be added, by way of summary, that shallow seas 

 free or nearly free from terrigenous sediment, and abounding in 

 lime-secreting life, furnish the conditions for nearly pure deposits 

 of limestone, and that most of the limestone within the areas of the 

 present continents appears to have originated under such conditions. 

 The common notion that limestone is normally a deep-water forma- 

 tion is an error. Although limestones are formed in deep as well as 

 in shallow waters, the more important classes of lime-secreting 

 organisms are limited- to the depths to which light penetrates. After 

 being formed, limestones may lose many of their original character- 

 istics, but enough usually remain to tell the story of their origin. 



Deep-sea Deposits 



Contrasted with shallow-water deposits. Deep-sea deposits 

 cover the ocean-bottom below the loo-fathom mark. Their area is 

 about two-thirds of the earth's surface. The characteristic deposits 

 are muds, organic oozes, and clays, which, in their physical 

 characteristics, are remarkably uniform. In regions of floating 

 ice, some diversity is introduced from the varied nature of the ma- 

 terials which it transports. 



