54 



wrought by two vertical shafts. In that nearest the outcrop, the 

 coke is reached at 112 feet from the surface, in the other at 207 

 feet, the dip of the coal measures being nearly west, and at a 

 low angle. A third shaft, recently wrought, which lies nearer 

 the margin of the basin, than either of the preceding, cuts the 

 stratum of coke at the depth of 90 feet. A bed of whin stone, 

 or coarse gray trap, is intercalated in the coal measures of this 

 part of the basin, intersecting the two first mentioned shafts, but 

 cropping out a little west of the third. This bed is met with in 

 the deepest and most western of the shafts, at a distance of about 

 100 feet from the surface, and is more than 30 feet thick where 

 it is cut through ; but in the next shaft, it is struck at a depth of 

 less than 30 feet, and has thinned down to about half the preced- 

 ing thickness. 



One of the most remarkable effects produced by this igneous 

 bed, is seen in the stratum of carbonaceous fire-clay which lies 

 next beneath. This, which in the second shaft has a thickness of 

 eleven feet, has been greatly indurated, and made to assume a 

 columnar structure, by which the whole mass is converted into 

 a congeries of closely packed five and six-sided prisms, often 

 quite regular, usually about half an inch in diameter, and always 

 at right angles to the lower surface of the trap. A portion of 

 this bed, originally occupied by impure coaly matter, presents the 

 same columnar structure, but the material is a compact plumba- 

 ginous coke, with much earthy matter intermixed. The general 

 aspect of the gray part of this bed strongly resembles that of 

 the coarser varieties of fire-brick, after they have been long 

 exposed to intense heat. This is what might be expected, for in 

 the bed in question, we have the very materials of fire-brick, 

 and in the overlying trap we have a source of igneous action, 

 which, in the originally molten condition of this substance, could 

 not fail to work great changes in the contiguous strata. This 

 columnar indurated clay, or natural fire-brick, when recently 

 broken, emits a most offensive odor, partly that of sulphuretted 

 hydrogen, and partly perhaps caused by a sulphuret of carbon. 



At the depth of about seventy feet below the bottom of the 

 trap, occurs the bed of natural Coke, for the mining of which 

 chiefly these openings have been made. This interval below the 



