MATERIALS OF THE EARTH 



37 



shells and other secretions are left behind. Shells and other secre- 

 tions of calcium carbonate, cemented together, make limestone, the 

 composition of which, when pure, is CaCO 3 . It is thought that 

 most limestone was formed of organic secretions, though this is 

 sometimes questioned. The shells, coral, etc., may or may not 

 have been fragmented before cementation. 



Limestone has many varieties. One variety is chalk (p. 82). 

 Again, magnesium may replace the calcium in various proportions. 



Fig. 15. Globigerina ooze. Magnified 20 times. (Murray and Renard.) 



If there is any considerable amount of magnesium, the rock is 

 called dolomite. The composition of ideal dolomite would be ex- 

 pressed by the formula CaMgCO 3 ; but the amount of magnesium 

 is variable. There was little magnesium in the shells, coral, etc., 

 out of which much limestone was made. Sometimes the dolomi- 

 zation of the limestone (the conversion of CaCO 3 into CaMgCO 3 ) 

 appears to have taken place long after the limestone was formed, 

 while in other cases it appears to have taken place while the ma- 

 terial of the limestone was being deposited. 



Siliceous deposits. In the decomposition of igneous rocks, a 

 little of the silica, as well as of the bases, is dissolved and carried 

 away in solution. Certain organisms extract this from solution 

 for their tests, shells, etc., just as others extract calcium carbonate. 

 The accumulation of siliceous secretions often forms siliceous rocks. 



