284 



measures, we 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 impregnated 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 car- 

 bonaceous matter. This is well shown on comparing 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 in large amount, both in the shape of 

 layers of 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 compara- 

 tively 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-Car- 

 bonate in the strata and the presence, either now or formerly, of 

 a large amount of carbonaceous or vegetable matter becomes 

 even more striking on a detailed examination of particular beds. 

 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 

 whh the impressions and carbonized remains of plants, are at the 

 same time more or less impregnated with this ferruginous com- 

 pound. 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 inclose thick courses of the lenticular ore, form any 

 exception to this law. For although in their present state they 



