612 E. BLACKWELDER — ORIGIN OF BIGHORN DOLOMITE OF WYOMING 



Partial Analyses of Dolomites from the Bighorn Formation 



12 3 4 5 6 7 



SiO, 2.30 



A1 2 8 and F^Oa 1.50 22 trace 



Insoluble in dilute 



HC1 (3.80) 4.67 .53 .35 .42 1.99 .0 



CaO 29.35 30.14 31.17 30.62 30.43 30.34 30.4 



Mg O.... A .20.19 18.41 20.76 20.32 20.85 20.10 21.9 



C0 2 45.00 44.24 47.75* 46.92* 47.10* 46.30* 47.7 



Undetermined 1.06 2.54* .00* 1.79* 1.20* 1.32* .0 



Total 100.00 100.00 100.43 100.00 100.00 100.05 100.00 



1. Massive cream-colored dolomite, Bull Lake, Wind River Range. Analyst, 



J. G. Fairchild. 



2. Thin-bedded gray dolomite of fossiliferous zone, Doubletop Peak, Gros Ventre 



Range. Analyst, C. Palmer. 



3. Massive cream-colored dolomite, Labarge Mountain, Green River Valley. 



Analyst, J. G. Fairchild. 



4. Massive gray dolomite, Shoal Creek (upper falls), Gros Ventre Range. 



Analyst, J. G. Fairchild. 



5. Massive gray mottled dolomite, Leigh Creek, Teton Range. Analyst, W. C. 



Wheeler. 



6. Average of analyses, 1-5. 



7. Ideal composition of dolomite. 



Under the microscope the rock appears to consist almost entirely of 

 carbonate crystals, which, of course, must either be those of the mineral 

 dolomite or a mixture of calcite and magnesite. Microchemical tests indi- 

 cate dolomite only. In a single instance a minute cavity was found to be 

 filled with chalcedonic quartz, and in some specimens the interstices be- 

 tween the dolomite crystals contain dustlike particles of red ferruginous 

 matter which gives the rock the pinkish tinge occasionally noted. It is 

 significant that in all the specimens'examined under the microscope not a 

 single sand grain has been seen, and particles of any kind, other than the 

 carbonates, are very rare. 



Texturally the rock shows considerable variety: some parts are fine 

 grained, some moderately coarse, and still others contain rather large 

 particles imbedded in a fine-grained mass. The darker parts consist of 

 very small, tightly interlocking crystals of dolomite averaging about .05 

 millimeter in diameter. The lighter portions of the rock which inclose 

 the dark bodies are decidedly coarser and also more variable in texture. 

 The average diameter of the crystals here is about .10 to .16 millimeter, 

 but many are less than .05 millimeter and others more than .3 milli- 



• Calculated. 



