AN UNDERGROUND EXCURSION 145 



are stalled and fed in side-rooms excavated in the coal and 

 superincumbent rocks. The requisite circulation of pure 

 air is maintained through the mine by the consumption of 

 refuse coal at some suitable place, the smoke and heated 

 air from which ascend through a separate shaft. The es- 

 cape of heated air through this shaft causes a descent of 

 external air to take place through the main shaft. Com- 

 munication between the two shafts is effected only through 

 the remote portions of the mine, so that the pure air is 

 made to permeate all the passages. Still there must al- 

 ways be side-rooms through which no circulation can be 

 effected, and here not unfrequently collects that explosive 

 " fire-damp," or light carbide of hydrogen, so often evolved 

 spontaneously from the coal, and so often the cause of fatal 

 accidents to the miners (Fig. 62). When the seam of coal 

 is less than five feet thick, it becomes necessary to remove 

 some of the superincumbent rock, to render the roofs of the 

 main passages sufficiently high for the mules to travel un- 

 der them. 



Thus entire square miles of a coal-seam, hundreds of feet 

 beneath the surface, are perforated in all directions by the 

 hand of the miner (Fig. 63), as ship-timber is riddled by 

 the depredations of the Teredo. 



By the feeble light of our miner's lamp we enter one of 

 these dusky aisles. The substratum beneath our feet has 

 been ground to dust. The whole thickness of the coal- 

 seam is exposed along the lateral walls. Occasionally it 

 presents gentle undulations instead of lying in a rigidly 

 plane position, and not unfrequently a huge bulge of the 

 underlying rocks completely cuts off the seam. Overhead 

 a black, bituminous shale forms the ceiling. Perhaps here 

 and there the white shell of a univalve or a bivalve pro- 

 jects from the surface— the products of the sea buried in 

 their native sediments, and suspended above our heads. 



G 



