358 The Philosophical Society of Glasgow. 



closes the whole gradation or gamut of metamorphism in a sequence 

 so full, so uninterruptedly progressive, and geographically so extensive. 

 Appealing to his geological map of the United States, and to several 

 transverse geological sections constructed from his own surveys, Professor 

 Rogers established this position, by pointing out the distribution of 

 all the respective kinds of coal, showing that the anthracite variety, 

 destitute of the hydro-carbon compounds, is restricted to the closely 

 compressed basins ranging along the south-east side or margin of the 

 Appalachian mountain-chain ; the semi-anthracite, with from 5 to 

 7 per cent, of hydro-carbon, to the middle zone of the same range; the 

 semi-bituminous coal, containing 15 or 20 per cent., to the north-west 

 margin of the moimtain belt ; and the richly bituminous or normally- 

 hydi'ogenous kinds, to the wide and very shghtly disturbed tracts 

 still more to the north-west, or beyond the mountains, and more interior 

 in the continent. 



Accompanying this gradation, which embraces between its extremes 

 a breadth of country of more than 200 mQes, there is traceable, pari 

 passu, an equally well marked transition in all the symptoms of meta- 

 morphism affecting the other strata, or those not including the coal 

 seams, and a gradation not less conspicuous in the flexures of the strata, 

 evincing a progressive abatement north-westward in the energy of the 

 subterranean forces which undulated them, and left the eastern half of 

 the coal region in the form of a series of long, slender, parallel troughs. 

 The main point of the author's paper was to show that the copious pre- 

 sence of gaseous hydro-carbon (illuminating gas), and hydro-carbon in 

 its liquid form (peti-oleum or rock oU), is simply one phase in this gra- 

 . dation, mai-king a particular stage in the metamorphic process, at which 

 the expulsion of the volatile constituents of the coal beds had ceased. 

 Appealing to the now well established fact, that within the great Appa- 

 lachian coal-field we can nowhere meet with any igneous rocks — no out- 

 pom's nor dykes of trap — no intrusive mineral veins — none even in the 

 most undulated and altered portions of the anthracite-yielding coal- 

 measures of Eastern Pennsylvania, nor, indeed, within twenty-five or 

 thirty miles of their border ; and referring to his repeatedly pubUshed 

 views of the volatilizing and discharging agency of subterranean steam, 

 issuing hot and dry through the crevices of the strata, during convul- 

 sive earthquake upheaval and oscillation of the coal measures, he infers 

 that the petroleum and burning gas of Western Pennsylvania and 

 Western Virginia were extricated or distilled from the already framed 

 hydrogenous coal seams, and merely not wholly expelled into the 

 atmosphere as in the anthracite region, but arrested in the pores and 

 crevices of the overlying strata at a stage short of their total expulsion. 



