BITUMINOUS AND ANTHRaCITIC. 441 



finer materials will subside, and in an increasing quantity, up to 

 a certain point, at which the loss of velocity in the current is com- 

 pensated by the exhaustion of material, when a gradual and final 

 thinning of the deposit will take place. 



If we examine, in the next place, the gradations of thickness 

 visible in the limestones and other marine deposits, they wall be 

 found to lead to precisely similar inferences respecting the posi- 

 tion of the ancient land. Viewed either together or individually, 

 the limestones of the coal-measut^^ of Pennsylvania, Virginia, 

 and Ohio, display a remarkably uniform augmentation, as we 

 trace them westward. Thus, throughout all the southeastern 

 basins, comprising the whole of the anthracite coal-fields of Penn- 

 sylvania, and the Broad Top mountain in the same State, the 

 formation exhibits a total absence of limestone, and a correspond- 

 ing deficiency of calcareous matter in the shales and the iron ores. 

 Advancing, however, a distance of twenty-five or fifty miles north- 

 westward, to the general southeastern margin of the great bitu- 

 minous region, where we enter on the first of the chain of partially 

 insulated troughs adjacent to the escarpment of the Allegheny 

 mountain, we no longer encounter a total poverty of limestone, 

 though we still meet with a striking deficiency. As an evidence 

 of this, let us take one of the basins of the Allegheny mountain, 

 that, for example, which lies near the head of the Potomac river. 

 The minute researches there made, in connection with the geo- 

 logical surveys of Virginia and Pennsylvania, have shown that 

 the total thickness of the limestones, counting all the thin bands and 

 layers of nodules, does not probably exceed ten feet. This state- 

 ment is confirmed by a pamphlet on the same coal-field, describing 

 the land of the Cxeorge's Creek Company, by Messrs. Alexander 

 and Tyson. In their very full section of the strata, we do not see 

 a single band of limestone introduced. 



Turning to the Moshanan basin, in Centre county, which is 

 also a marginal trough of the great western coal-field, the entire 

 quantity of limestone appears to be about seven or eight feet. If, 

 however, we pass westward from this southeastern line, and cross 

 the great coal-field by any section, between the Susquehanna 

 in Pennsylvania, and the Little Kenawha in Virginia, we witness 

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