104 MINERALS OF MARYLAND. 



opaque or faintly translucent, and varies in colour from brown 

 to dull yellow. The sulphuret of ii^on is either crystallized 

 or takes the form of wood, the structure of which may be 

 seen. 



In that part of the district nearest the primary rocks argil- 

 laceous carhonate of iron exists in abundance. It is in the 

 form of nodules, varying from a few inches to several feet in 

 diameter, of a gray colour, and has a compact structure when 

 it does not embrace sand. The cavities of the nodules are 

 often lined with crystals of pure carhonate of iron, which in 

 most instances are so small and confused that their form 

 cannot be determined ; they approach nearest to that called 

 mixte by Haliy. The composition of these nodules in many 

 localities is gradually changed into hydroxide of iron; by 

 acquiring an additional portion of oxygen and combining with 

 water. The carbonic acid being liberated forms bicarbonates 

 of lime, and magnesia or manganese, if these earths be 

 present. The soluble bicarbonate of lime, meeting occasion- 

 ally with sulphate of iron resulting from the oxidation of 

 pyrites, which is sparingly found in the iron ore deposites, 

 produces the small crystals of sulphate of lime occasionally 

 observed in the cavities of the nodules. The structure of the 

 nodules is changed as well as the composition, but the ex- 

 ternal form remains unaltered. Tlie oxidation commencing at 

 the surface and proceeding inwards produces a series of con- 

 centric layers. The composition of the crystals is also 

 changed, so as to present us with hydrated peroxide of iron, 

 whose crystalline form is that of the carbonate of protoxide. 



These ores were extensively smelted before the revolu- 

 tionary war, and the greater part of the iron exported to 

 England. They now furnish to three furnaces the material 

 for producing a very superior metal. The scarcity of fuel 

 prevents their being more extensively worked.* 



The second division consists of primary rocks, and ex- 

 tends north-westerly from the first to Parr'^s spring ridge y 

 where the rocks begin to assume the character of transition. 

 This ridge passes through the northeast part of the new county 

 of Carroll, and running south-southwest reaches the Potomac 



* Bruce's Mineralogical Journal, vol. i. p. 323. 



