I30 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. 



90. Dolomite [(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. 



91. 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. 



93. SiDERiTE AND Spiiaerosiderite [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. 



Sphaerosidcrite 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 micro- 

 scopic. For example : in the olivine diabase of Campton falls, the little 



