GEOLOGY. 321 



very material 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 co- 

 lumnar 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 fire-clay is occupied by bluish 

 and drab argillaceous and sandy slates, with some coarse sandstone, the 

 former abounding in impressions of plants, among which may be noted 

 Equisetum columnare, Zamites oblusifolius and Tteniopteris magnifolia, 

 forms which many years ago Prof. Rogers pointed out as marking the 

 Oolite age of these coal-bearing strata. The baking action of the trap is 

 curiously shown in all these fossils. The coaly matter of the stems and 

 fronds, when closely examined, is seen to be blebby or blistered. It is, in 

 fact, coke, which, while it retains the outlines and stronger markings of 

 the plant, has, in its partial fusion, obliterated all the finer characters of 

 the organized surface. 



The coke, where it has been successfully mined, forms a bed about five 

 feet thick, including but little slate, and presenting a nearly homogeneous 

 mass -of a bluish-black color, uniformly vesicular, and light enough to float 

 in water. It retains only a minute fraction of the volatile ingredient of 

 the unaltered bituminous coal of this region, but it ignites readily, and 

 burns like the compacter kind of ordinary coke. Throughout the bed, 

 but especially towards the top, it presents a partially columnar structure. 

 Where this structure is marked, the coke is found to crepitate when 

 heated. In some localities on the south side of the James River, where the 

 whole mass of coal and adjoining shale has been rendered completely col- 

 umnar, the material in the process of heating breaks up with explosion 

 like the crack of a pistol, at the same time projecting its fragments to 

 some distance from the grate. 



The gradually diminishing influence of the trap bed, as we recede 

 downwards, is illustrated by the section in'one of the shafts, which embraces 

 a thickness of fifty feet of strata below the seam of coke above described. 

 After passing through indurated fire-clay, lying immediately beneath the 

 coke, we have a thickness of about twenty feet of slates, followed by a 

 thin seam of semi- coke, or coky coal, more bituminous below than at 

 top ; and after this, descending through some twenty feet more of 

 slates and sandstones, we come upon a bed of bituminous coal, which ap- 

 pears to have sustained no alteration beyond the development throughout 

 the mass of a columnar structure. In the deepest of the three shafts, the 

 seam, now wrought under the intelligent direction of Col. Worth, cor- 

 responds to the coky coaL above described, the lower layer retaining much 

 of its original bitumen. In all these workings the gradation of metamor- 

 phic influence is beautifully marked within a distance of less than fifty 

 H* 



