EVOLUTION OF LIMESTONE AND DOLOMITE 



333 



that the transported material contained progressively larger 

 proportions of magnesium as its distance from the parent lime- 

 stone increased. Near its point of origin, the marl carried 3 . 7 

 parts of magnesium carbonate to 100 of calcium carbonate, and 

 from these figures the ratio was generally raised to 36 magnesium 

 carbonate and 100 calcium carbonate. 



TABLE II 



Table Computed from 21 Globergerina, 21 Red Muds, 7 Radiolarian Oozes, 

 Pteropod Ooze, Diatom Ooze, and Blue Mud 



* Relative values express parts of MgC0 3 : ioo parts of CaC0 3 . 



The stalactites from the caverns in the coral rocks of Bermuda, 

 according to Hogbom, contain only o. 18 to 0.68 per cent of mag- 

 nesium carbonate, while the rocks themselves carry about live 

 times as much magnesium carbonate. The lime salt has evidently 

 dissolved much more freely than the magnesium compound. 

 This mode of concentrating magnesium carbonate must, from 

 the evidence cited, be regarded as real, and may be instrumental 

 in causing the development of dolomite. It is obvious that the 

 smaller the content of calcium in the sea, as compared with mag- 

 nesium, that is, the smaller the relative quantity of calcium result- 

 ing from the difference between contribution from the land on the 

 one hand, and precipitation on the other, the more effective will 

 be the process of leaching. The precipitation of calcium carbonate 

 in the sea is now largely controlled by life processes, and probably 

 has been at least as far back as the beginning of the Paleozoic, 

 so that since Zoic times there has been a powerful agency at work, 

 tending to deplete the calcium content of the sea, while leaving 

 the soluble magnesium salts to accumulate. Solid phases of 

 calcium carbonate are therefore farther removed from a condition 



