.^rA^NKl^ ok ]\rATJiNi': ixjT-omttiza^iion 44o 



CRYSTALLIZATION OF ])OJ)OM/TfJ Fh'OM HlJA-WATEh'S PERMEATING THE 

 SEDIMEXTS OF THE SEA-BOTTOM 



The buiicliv, irre<>-iilar distribution of dolomite o-rains in mixed beds ol 

 limestone and dolomite, as sliown by stains, may be due in part to the 

 direct crystallization ol' dolomite grains from waters in the ooze of the 

 sea-bottom. The inclosure of dolomite grains in very line-grained, com- 

 pact calcite masses (also shown by plate 22) suggests the same thing. 

 Proof of this process was not obtained from the samples studied. Skeats 

 fomid dolomite in druses and vugs of recently uplifted coral reefs of the 

 southern Pacific coral reefs, indicating that direct crystallization of dolo- 

 mite from sea-waters does take place under certain conditions. Certain 

 dolomite rhonibohedra in the oozes of the Mediterranean^^ may Ije due to 

 the same ])rocess. 



DIRECT PRECIPITATION OF DOLOMITE FROM THE SEA 



The grain of some dolomites is so line (less than .01 mm.) as to sug- 

 gest that they are chemical precipitates. The relatively large size of the 

 dolomite grains in mixed beds of dolomite and calcite where the dolomite 

 is clearly a replacement adds weight to the view that the fine-grained 

 dolomite beds are of a different origin. In respect to grain, the fine- 

 grained dolomites resemble the bacterially precipitated lime carbonate 

 oozes of modern seas. Because of the FeO content of dolomites, it is 

 improbal)le that any of them were thrown out of the upper strata of sea- 

 water, but only in the lower layers near the bottom wliere reducing condi- 

 tions would be probable. 



COXDITIOXS or DOLOMITIZATIOX IX THE SeA 

 CRITICAL STATE BET^\EEN LIMESTONE AND DOLOMITE DEPOSITION 



The doininance of pui'e limestone and dolomite over mixed Ixxls of the 

 two, as showji by staijis and analyses, and the alternations ol" fairly pure 

 limestone and dolomite l.^eds indicate that the critical state between lime- 

 stone and dolomite depositing conditions is a narrow one, and that when 

 the borderland between the two is reached moderate changes, as a rule, 

 halt deposition of one kind and substitute another. Special conditions 

 may at times ha^•e tempered dowji the full effectiveness of dolomite-pro- 

 ducing conditions. In many mixed beds it is evident that the calcite 

 represents compact residuals which escaped dolomitization because of 

 their impermeable condition. For exam])le, a certain l)ed in the Galena- 



1*0. B. Boggild : Report of the Danish Gcograpliical Expedition, lOOS-lOJO. 



