130 MINERALOGY AND LITHOLOGY. 
impurity, in some of our limestones the analyses show a greater or less 
replacement of the calcium by magnesium. By this replacement, calcite 
approaches dolomite. 
go. DotomitE [(Ca, Mg) CO*)]. 
Pure dolomite is not very common in our state, though the limestones 
very often contain more or less magnesia, It exists in considerable quan- 
tities at Lyman, and also at Plainfield. It is readily distinguished, be- 
cause it does not effervesce in cold, diluted hydrochloric acid. Our 
dolomites are gray, and quite impure. Dr. Jackson’s analysis of the 
Plainfield dolomite indicates the presence of about thirty per cent. of 
impurities, which are of mica, quartz, and other silicates. In thin sec- 
tions, it does not show the twin laminations in polarized light that are 
shown by calcite. 
gt. ANKERITE [(Ca, Mg, Fe, Mn) CO*]. 
This mineral, in which the magnesia of a dolomite is more or less 
completely replaced by iron and manganese, is commonly present in the 
quartz veins that have been shown to be auriferous, and is, indeed, char- 
acteristic of them. It is found in good rhombohedral crystals of a honey- 
yellow color, and on heating them in the reducing flame of the blow-pipe 
they become magnetic. Littleton, Lisbon, and Lyman are localities where 
they are abundantly found. In some veins the quartz, though containing 
no ankerite, is filled with rhombohedral cavities, showing that there once 
were crystals that have been dissolved away. 
92. SIDERITE AND SPHAEROSIDERITE [Fe CO?]. 
The carbonate of iron, as has been before noted, is common in the 
deposits of bog-iron ore, but not as a mineral of interest, since its pres- 
ence is only shown by the effervescence that takes places when they are 
treated with acid. Near us, at Plymouth, Vt. there are deposits of sid- 
erite. 
Sphaerosiderite is a concretionary variety of siderite that is found in 
globular or mammillary forms. This mineral is quite often found as a 
constituent of our rocks, as a lining of cavities; but it is chiefly micror 
scopic. For example: in the olivine diabase of Campton falls, the little 
