440 E. STEIDTMANN— ORIGIN OF DOLO^^IITE 



waters. Uiidergroimd waters of deserts and of great depths^ however^ 

 often resemble sea-waters in concentration. 



5. Many Tertiary coral reefs of the southern Pacific are more or less 

 dolomitized. Here the only possible metasomatic agent was sea-water. 



Manner of Marine Dolomitization 



the processes 



It is conceivable tliat marine dolomites may have resulted from one or 

 nioi'e of the following processes : 

 1-Jeplacement of lime carbonate; 



l^ecrystallization of MgO-bearing aragonite and calcite skeletons: 

 Crystallization from water permeating the sediments of the sea-])ottom : 

 And direct chemical precipitation. 



DOLOMITIZATION IIY REPLACEMENT OF LIME CARBONATE AT THE 



SEA-BOTTOM 



Evidences of replacement. — Many facts argne for the development of 

 marine dolomites by the replacement of lime carbonate. Some prove it. 

 Others fit more than one interpretation equally well. Evidence for this 

 process obtained hy direct stndy of sami)les is presented under the fol- 

 lowing headings: 



l^elations of dolomite gi'aiiis to the l)e(hliiig. 



Isolations of dolomite to fossils. 



lielations of dolomite grains- to each other and to calcite grains. 



Eelations of dolomite to pervions marine structures. 



Relations of dolomite grains to llie bedding as slioun hij staifis. — 

 Plates 23 and 24 show a typical mixture of primary calcite and dolomite 

 ditferentiated by stains. The l)ed(]ing is horizontal. The bunchy^ irreg- 

 ular distribution of the dolomite proves that it was formed chiefly bv 

 reactions within the sediments and not by direct precipitation, '['he latter 

 process would have caused a more even arrangement of tlie dolomite 

 particles. The existing relation of the dolomite grains respective to tlie 

 bedding may be due to replacement of lime carbonate and to crystalliza- 

 tion of dolomite from the sea-waters in the ooze. Other facts gi\'e stress 

 to the process of replacement. 



Relations of dolomite to fossils. — Well preserved fossils are less abun- 

 dant in dolomites than in limestones. Silicified forms, however, ai-e as 

 a rule very fine. Microscopic fossils were rare in the dolomite samples 

 studied, but the limestones were usually full of them. AYeller^^ found 

 the fossils in dolomite to be about the same in kind and development as 



i"Stnnvt Wollci-: P.uU. Oool. Soc. Am., vol. 22, 1911. 



