GEOLOGY. 315 



may remark certain general conditions connected with the abundance or with 

 the comparative absence of the proto-carbonate in the strata. 



One of these is seen in the fact that the lenticular ores and strata, impreg- 

 nated with proto-carbonate of iron, are in a great degree restricted to such 

 divisions of the carboniferous rocks as include beds of coal, or are otherwise 

 heavily charged with carbonaceous matter. This is well shown on compar- 

 ing together the four subdivisions of the carboniferous rocks of the great 

 Trans- Alleghany coal region, as classified under the head of the Serai coal 

 series of the Pennsylvania and Virginia Geology. In the first of these, 

 designated as the Older coal measures, the proto-carbonate is found hi large 

 amount both in the shape of layers and lenticular ore, and diffused through 

 the substance of the shaly strata. In the next division above, distinguished 

 as the Older barren shales, and which, as the name implies, is comparatively 

 devoid of carbonaceous matter, much less of the proto-carbonate is met with. 

 In the third group, that of the Newer coal measures, the ore again abounds, 

 and in the uppermost division, or Newer barren shales, it has a second time 

 almost disappeared. 



The connection between the development of the proto-carbonate in the 

 strata and the presence, either now or formerly, of a large amount of car- 

 bonaceous or vegetable matter, becomes even more striking on a detailed 

 examination of particular heads. Thus, in the coarse sandstones of the coal 

 measures which are comparatively destitute of vegetable remains, we find 

 little admixture of the proto-carbonate. On the other hand, the fine grained 

 flaggy argillaceous sandstones, which are often crowded with the impressions 

 and carbonized remains of plants, are at the same time more or less impreg- 

 nated with this ferruginous compound. So, again, the soft argillaceous 

 shales, in the midst of which the lenticular ore so frequently presents itself, 

 show, by their dark color and included impressions of plants, as well as by 

 actual analysis, that they are richly imbued with vegetable matter. Nor do 

 the nearly white fire-clays, which in many cases enclose thick courses of the 

 lenticular ore, form any exception to this law. For, although in their 

 present state they contain little or no carbonaceous matter, the marks of 

 innumerable roots of Stigmaria and parts of other plants which everywhere 

 penetrate the mass, show that at one time they must have been crowded 

 with vegetable remains. 



A further and yet more striking proof of the influence which the con- 

 tiguous vegetable matter has had in the formation of the proto-carbonate, is 

 seen in the fact that the most productive layers of the ore are commonly met 

 with quite near to the beds of coal, and that frequently crowds of the 

 nodules are found in the carbonaceous shales or partings which lie in the 

 midst of the seam itself. 



"While the strata, including the proto-carbonate, are thus distinguished by 

 the admixture of more or less carbonaceous matter, they are also remarkable 

 for seldom exhibiting a distwctty red tint. Presenting, where not weathered, 

 various shades of greenish-grey and olive, and bluish-black, they only 

 become brown or red where, by exposure to air, the proto-carbonate has 

 been converted into the sesquioxide of iron. On the other hand, those 



