EVOLUTION OF LIMESTONE AND DOLOMITE 339 



B. THE EVIDENCES FOR THE ORIGIN OF DOLOMITE BY THE META- 



MORPHISM OF LIMESTONES AFTER THEIR EMERGENCE 



FROM THE SEA 



Direct chemical precipitation of dolomite. — The occurrence of 

 dolomite in veins and vugs of limestone and dolomite is well known. 

 So far as the recorded facts seem to indicate, direct chemical 

 precipitation of dolomite in fissures and openings of carbonate 

 formations, after emergence from the sea, seems to be much more 

 important than the chemical precipitation of dolomite in the sea 

 at the present time. However, dolomite as a cement and vein 

 filler is quantitatively of far less consequence than calcite. Fur- 

 thermore, it is not certain to what extent dolomite in veins merely 

 i means the transfer of previous existing dolomite originally developed 

 in the sea. Direct chemical precipitation of dolomite in formations 

 after their emergence from the sea, while probably more important 

 than the chemical precipitation of dolomite in the sea at the present 

 time, is nevertheless a very subordinate process and cannot be 

 regarded as an important source of dolomite formations. 



Origin of dolomite by leaching limestones after their emergence 

 from the sea. — The dolomitization of limestones after their emer- 

 gence through leaching is a very important process wherever both 

 the calcium carbonate and dolomite are originally present, because 

 of the differential solubility of the two carbonates. Unquestion- 

 ably a unit volume of water as it enters the soil, but charged with 

 atmospheric gases, is a much more efficacious solvent than an 

 equal volume of sea water, but the total volume of sea water which 

 is effective in leaching carbonates in the sea is vastly greater than 

 the volume of water in the lithosphere. Leaching of limestones 

 by underground waters probably changes the ratio of calcium to 

 magnesium but little excepting near the surface, because the 

 porosity, slumping, and faulting which would need to follow from 

 converting a magnesian limestone containing even 13 per cent 

 of magnesium carbonate into a dolomite through leaching is 

 not at all evident in dolomite formations. Evidence is cited 

 showing that weathering does increase the percentage of 

 magnesia in limestones. Since effective leaching of limestones 

 by ground water is related to weathering, it is improbable that 



