458 ORIGIN OP THE APPALACHIAN COAL STRATA, 



Appalachian coal-formation, in which this remarkable contiguity 

 of marine calcareous strata and vegetable or teiTcstrial coal occurs, 

 is the great western basin of the Allegheny and Ohio rivers. I 

 haye already mentioned the abundance of unquestionably oceanic 

 limestones in this coal-field, and given my inferences from the 

 interesting fact, that they augment in thickness, and multiply in 

 number, in crossing the region northwestward. As, however, the 

 actual contact of beds of coal and limestone is of rare occurrence 

 in the coal-fields of other countries, and as the circumstance must 

 have an influential bearing on all our speculations concerning the 

 physical conditions prevaUing at the formation of the strata, and, 

 to a certain extent, on our whole theory of the origin of coal, I 

 shall here describe some of the best known instances before I 

 reason concerning them. 



Confining our attention to the great western basin, where the 

 most strildng cases occur, the following instances of this contact 

 present themselves, in the ascending order. 



1st. In the lower division of the main coal-measures, there 

 occurs, near the town of Mercer, in Pennsylvania, a seam of good 

 coal, having a thickness of about two feet, which is immediately 

 overlaid by a bed of very pure limestone, also about two feet 

 thick, containing a variety of marine organic remains of the gen- 

 era Terebratula, Bellerophon, &c. In some spots, the pure coal 

 is not separated from the pure limestone by more than a single 

 inch, or at most two inches, and then the interval is filled with a 

 calcareo-carbonaceous shale. 



2nd. Higher in the series, but still in the lower part of the 

 main coal-measures of western Pennsylvania, we meet with a bed 

 of fossiliferous limestone, the thickness of which, in many neigh- 

 borhoods, near the Allegheny river, is about fifteen feet. It con- 

 tains several oceanic species, among them some Crinoidecp^ two 

 species of Terebratula, and a Goniatites. Li some places, this 

 stratum embraces a thin seam of cbal, four inches thick, in almost 

 direct contact with the limestone. 



3d. The limestone, which is the first underneath the Pittsburg 

 seam, contains a bed of coal one foot in thickness, separating two 

 of its lower layers. 



