55 



fire-clay is occupied by bluish and drab argillaceous and sandy 

 slates, with some worn sandstone, the former abounding in im- 

 pressions of plants, among which may be noted Equisetum co- 

 lumnare, Zamites obtusifolius, and Tseniopteris magnifolia — 

 forms which, many years ago, Prof. Rogers pointed out as mark- 

 ing 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 organ- 

 ized 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,i uniformly 

 vesicular, and light enough to float in water. It retains only 

 a minute fraction of the volatile ingredients of the unaltered 

 bituminous coal of this region, but it ignites readily, and burns 

 like the compactor kinds of ordinary coke. Throughout the 

 bed, but especially towards the top, it presents a partially co- 

 lumnar 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 columnar, the 

 material, in the process of heating, breaks up with explosions 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 in- 

 durated 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 the 

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

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

 which appears to have sustained no alteration beyond the de- 

 velopment throughout the mass of a columnar structure. In the 

 deepest of the three shafts, the seam now wrought under the 



