GEOLOGICAL SOCIETY OF PENNSYLVANIA. 



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same time of maintaining in the plane of its own superior 

 surface a certain general parallelism or level. They 

 thus furnish a ready explanation as to the otherwise sin- 

 gular irregularities in the thickness of the great lower 

 coal seam, so different from the usual characters of coal 

 veins, as they are termed, in the secondary formations. 



After this coal has been excavated, and its base has 

 been laid bare, the floor of the mine exhibits the original 

 undulating surface of the granite. In some few instances 

 the eminences which protrude from this ancient surface 

 rise entirely through the lower carboniferous bed, as on 

 a larger scale an island rises above the waters of an ocean, 

 and the coal, in mining phrase, is ^' nipped out.'' Occa- 

 sionally these granite points or " walls" shut up all fur- 

 ther working in that direction. Drifts may be carried 

 round them, or by great labour and perseverance they 

 may sometimes be surmounted, when a descent is fre- 

 quently made into a body of good coal, filling a deep 

 hollow from 40 to 50 feet. These of course are the 

 extreme cases. 



Thus the floors of the deep galleries present an irre- 

 gularly undulating surface, to the inconvenience and re- 

 tardation of the miners, and occasioning frequent obsta- 

 cles to the formation of subterranean railroads. From 

 the lateral galleries the coal is dragged in basket wagons 

 by negroes, who pass over the deeper hollows on rough 

 wooden bridges and inclined planes. 



It is not always that this coal rests immediately upon 

 the granite. Occasionally there is a foot or two of black 

 carboniferous shale, or a seam of from one to six inches 

 of dark porphyritic rock. 



Often many square feet of the granite are laid bare, 

 when its superior surface appears smooth, as if worn by 

 previous attrition before the deposition of the vegetable 

 matter had commenced. 



As may readily be inferred, there exists no corres- 



