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entirely absent. But the remains of fish, and impressions of 

 carbonized parts of plants, occurring in this group of deposits, 

 are found embedded in layers of greenish and olive sandstones 

 and dark bituminous shales. So, in the southern parts of the 

 belt in Virginia and North Carolina, where these rocks include 

 seams of coal and extensive beds of sandstone and shale con- 

 taining the remains of plants, the usual red color is found to give 

 place to the gray, olive, and dark tints of the old coal measures, 

 and layers of Proto-Carbonate of Iron show themselves in the 

 vicinity of the coal seams. 



Taken in mass, the red and mottled strata of the unproductive 

 coal measures, or of the other groups of red rocks above alluded 

 to, would no doubt be found to contain, in an equal thickness, as 

 large an amount of Iron as the Coal-bearing strata which include 

 the layers of Carbonate ; the difference being that, in the former 

 case, the metal remains for the most part diffused through the 

 rock as a Sesquioxide, while in the latter, having assumed the 

 condition of Proto-Carbonate, it has to some extent been con- 

 centrated in particular layers or strata. According to a rough 

 estimate of the amount of Carbonate ore included in the lower 

 coal measures of the Laurel Hill region of Virginia and Penn- 

 sylvania, derived from a detailed examination of the ores and 

 associated strata at several points, it may be safely assumed that 

 the equivalent of Sesquioxide of Iron would not amount to one 

 third of one per cent, of the whole mass of this portion of the 

 coal measures, and a proportion not exceeding this is deducible 

 from the measured sections of ore and accompanying rocks in 

 the carboniferous strata of other tracts subjected to a similar 

 calculation. 



But even allowing a quantity three times as great as this, to 

 cover the diffused carbonate and the oxide in some cases mingled 

 with it, we should have only about one per cent, to represent the 

 proportion of ferruginous matter in the entire mass ; an amount 

 undoubtedly much less than exists in many of the strata of 

 red and purple shales and shaly sandstones of the carboniferous 

 series or of the groups of red rocks geologically above or 

 beneath it. 



In attempting to explain the origin of the Proto-Carbonate, 



