JNTKODrCTTOy 18B 



Wisconsin^ for opportunities of investigation. The metliod followed most 

 intensely was that of differentiating calcite and dolomites by stains. In 

 the field: were noted the relations of dolomite to limestone beds, the 

 textures and structures of dolomite, and the cliaracteristics of its fossils, 

 AV'hile in the laboratory the relations of calcite and dolomite grains to the 

 bedding, pervious structures, and to fossils were examined in thin-sections 

 ,and on polished slabs with the aid of staius. 



Staining methods for distinguishing calcite and dolomite, although 

 described in petrographic texts, are not in common use, because they are 

 generally thought to involve too })a in staking a technique to reward the 

 effort. Contrary to his expectations, the writer found them easy to 

 manipulate after experience suggested certain modifications, and so inter- 

 esting that a statement regarding their nature and efficacy seems justified. 



PRINCIPLES UNDERLYING CALCITE AND DOLOMITE STAINS 



The efficacy of stains for distinguishing calcite and dolomite is based 

 on two facts. (1) Calcite has difi'erent chemical properties than dolo- 

 mite. Certain reagents which react with dolomite either do not react 

 Avith calcite or else react at a different rate. (2) Complete mineral grada- 

 tions between calcite and dolomite are lacking. If isomorphism between 

 the two minerals was complete, staining capacity would not be a sharp 

 index of composition. 



FACTS SHOWING THE LACK OF COMPLETE ISOMORPHISM BETWEEN CALCITE 



AND DOLOMITE 



The MgO^ content of calcite crystals rarely exceeds 2 per cent. In 

 dolomite crystals CaO is nearly constant, but MgO is commonly replaced 

 by small amounts of FeO or of FeO and MnO. The FeO limit is about 

 10 per cent, but is rarely reached, it seems, excepting in Precambrian 

 sediments and in vein deposits. IModern organic calcium carbonate secre- 

 tions, although crystalline, often show a high MgCO., content. Calcific 

 echinoderm* skeletons from tropical seas are known to contain up to 13 

 per cent MgCOo, apparently in isomorphous union with calcite. Thin- 

 sections of Isocritius-' decorus off Havana, Cuba, with 11.42 per ceut 

 ^IgCOo, examined by the writer reacted homogeneously to stains and acids 

 as calcite. 



3 For analyses see Doeltor's Haiidbuch der Mineralchemie, Bd. 1, 2. p. 270, and Bd. i. 

 '.i, pp. 360-383. 



* U. S. Geol. Survey. Prof. Taper 9U-D. p. 34. 



•"• Specimens obtained rhroiiKh tlie courtesy of Austin FI. Claris, National Museum, 

 Washington, D. C. 



