EVOLUTION OF LIMESTONE AND DOLOMITE 337 



Experimental evidence on the origin of dolomite by secondary 

 replacement of calcium chloride. — -A summary of the principal 

 experimental methods by which the dolomitization of calcium 

 carbonate by replacement has been effected follows: 



I. REPLACEMENT AT HIGH TEMPERATURE 



By heating calcium carbonate and a solution of magnesium 

 sulphate in a closed tube at 200 C. (Morlot, Jahresb. Chemie 

 [1847-48], 1290). 



By heating carbonate of lime to over ioo° C. with a solution of 

 magnesium bicarbonate (F. Hoppe-Seyler, Zeitsch. Deutsch. Geol. 

 Gesell, XXVII [1875],' 509). 



By the action of a solution of magnesium chloride on calcium 

 carbonate at high temperature (Marignac and Faber, Compt. 

 Rend., XXVIII [1849], 364)- 



By saturating chalk with a solution of magnesium chloride and 

 heating the mass on a sand bath (Saint-Claire Deville, Compt. 

 Rend., XLVII [1858], 91). 



By heating fragments of porous limestone with dry magnesium 

 chloride in a gun barrel (J. Durocher, Compt. Rend., XXXIII 

 [1851], 64). 



It is probable that the preceding experiments on the replacement 

 of calcium carbonate have very little significance in relation to 

 the development of dolomite in the sea, since the continuity of 

 the life record back as far as the pre-Cambrian, at least, precludes 

 the possibility of a universally hot ocean. Very locally, as in the 

 case of the eruption of Krakatao, the required temperatures may 

 have been duplicated in the ocean. They are more likely to have 

 been paralleled in some cases of contact, metamorphism of lime- 

 stones generally following their emergence from the sea. 



2. REPLACEMENT ABOVE 6o° C 



By action of magnesium sulphate on aragonite in a concentrated 

 solution of common salt at a temperature above 6o° C. (Klement, 

 Min. Pet. Mitth., XIV [1894], 526). 



The same objection holds against the application of this experi- 



