Foote and Bradley — Solid Solution in Minerals. 343 



probable that there may have been a small amount of admixed 

 dolomite as that the calcite itself contained this unusual 

 amount of magnesium carbonate. The magnesium content of 

 our No. 6 is lower than any of the others. In this case, the 

 calcite was probably unsaturated. The crystal was a water- 

 clear specimen projecting two or three centimeters from a 

 mass of dolomite crystals which served as its base. The asso- 

 ciation was not an intimate one and the solution depositing 

 calcite had evidently not become saturated with magnesium 

 carbonate from the dolomite. The analysis is not given to 

 show the limiting value but to show that fairly intimate asso- 

 ciation of the minerals is necessary in order that the limiting 

 value may be reached. It is perhaps worth mentioning here 

 that iron in appreciable quantities was not found in any calcite 

 examined. No other samples of calcite were analyzed. 



Dolomite. 

 The specimens of dolomite deposited on calcite were the 

 following : 



I. A crystalline layer of dolomite deposited on massive cal- 

 cite. Guanajuato, Mexico. 



II. Small rhombohedral crystals of dolomite deposited as a 

 crust on scalenohedrons of calcite. Guanajuato, Mexico. 



III. An occurrence similar to that of No. II. On the 

 dolomite, calcite was subsequently deposited (see calcite No. 2). 

 Ouray, Colorado. 



The analyses are given in Table II. 



Table II. 

 Analyses of Dolomite deposited on Calcite. 



I II 



Sp. gr. 2-865— 2-914 Sp. gr. 2-891—2-907 



Guana jtiato, Mexico Guanajuato, Mexico 



a b Average a b Average 



MgC0 3 33-52 33-48 3350 31*50 31"45 31-48 



FeC0 3 6-14 5-95 6-05 8'43 8'49 8'46 



CaCO _. 60-68 60-61 60-64 60'67 60-76 60*71 



100-34 100-04 100-19 100-60 100'70 100'65 



III 



Sp. gr. 2-887—2-860 

 Ouray, Colorado 



a b Average 



MgC0 3 30-57 30-85 30-71 



FeC0 3 3-45 3-40 3-43 



MnC0 3 3-41 3-07 3-24 



CaCO 63-03 63-04 63'03 



100-46 100-36 100-41 



