THE WORK OF THE OCEAN 325 



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 in the 

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

 dolomite, composed partly or wholly of the double carbonate of 

 lime and magnesia (CaMg)CO 3 , abounds. This appears to have 

 been formed by a gradual substitution of magnesium for calcium, 

 in calcium carbonate, but just how and when and why the substi- 

 tution was effected is not fully understood. 



The plants and animals of the sea secrete notable quantities of 

 silica. Deposits of this sort are relatively more important in the 

 deep sea than in shallow water, and will be mentioned in that con- 

 nection. 



Limestone. Something concerning the origin of limestone has 

 already been given in the preceding paragraphs; but because of the 

 importance of this formation, 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 formation is a serious 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. In the shallow waters, these 

 plants and animals are in part free and in part attached. Within 

 the areas of deep water, they are free and at the surface, and their 

 remains drop to the bottom if not sooner dissolved. But few 

 forms live on the deep, dark, cold bottoms of the abysmal depths. 

 Clear waters, free from abundant terrigenous sediments and 

 abounding in lime-secreting life, rather than deep waters, are, 

 therefore, the most favorable conditions for the origin of lime- 

 stone. 



The purely chemical deposits of limestone are probably all of 

 shallow-water origin. Once made, they are subject to solution, 

 re-deposition, and other mutations like other deposits. As a result, 



