202 



ECONOMIC GEOLOGY 



transition of the associated limestones to iron ores traceable 

 both in the field and in the chemical laboratory. (5) The absence 

 of crumpling and shrinking of the strata, which points to a relative 

 rather than an absolute enrichment of the ores (Fig. 106). 



There are many widely scattered occurrences of limonite along 

 the Appalachian belt. These are especially abundant in Ala- 

 bama, Georgia, Tennessee, Virginia, Pennsylvania and Vermont. 

 The ores are often of uncertain or variable composition on account 

 of the admixed clayey matter, silica, and in some instances 

 vegetable matter. Beds of bog iron ore or limonite have been 

 worked near Brandon, Monkton and Bennington, Vermont, and 

 in several other localities in the southern part of the state. 



Brown or* deposits Hematite deposits Mago 



_ 



Contact of crystalline rocks Contact of crysta 



and Pakczoic sediments and coastal plain de 



FIG. 106. Map showing location of iron-ore deposits in Virginia. 

 After Harder. (By permission of the Macmillan Company, from Ries' 

 Economic Geology.) 



They were at one time of considerable commercial significance. 

 The iron ores of Brandon were discovered in 1810 and soon after 

 that some iron of superior quality was manufactured for several 

 years. The limonites of Alabama and Virginia furnish more 

 than 50 per cent, of the hydrated iron oxides in the United 

 States. 



The more important limonites are residual deposits forming 

 two distinct types of ore bodies. 



Residual Limonites. These result from the weathering of 

 vast areas of ferruginous rocks and the deposits appear as masses 

 in the residual clays. Limonites may also occur as bedded de- 

 posits, replacement deposits, and as bog ore deposits in swamps 



